Rogue One had multiple never-before-seen Imperial Droids, so what happened to them between Rogue One and Star Wars IV? Episode IV starts exactly where Rogue One ends as far as I know, so why suddenly no Imperial KX-series Security Droids? I want the in-universe reason, not just 'Because Episode IV was filmed about 30 years ago'.
According to the story, why are there no Imperial Droids like K-2SO in the original trilogy? Why did they all disappear all of a sudden after Rogue 1?
Most of the answers here so far have not focused much specifically on the class of droid that K-2SO belongs to, the KX-series Security Droid (also known as "Imperial enforcer droids"), which is what sparked this question pertaining to their presence (or lack thereof) during the events of the Original Trilogy. Sadly, while it is a good question, it is admittedly a difficult one to answer, as these droids are found solely in Canon materials (therefore, there is nothing in Legends to draw upon for this specific type of droid) and their appearances in Canon are currently quite limited (almost all of it pertaining to Rogue One).
Security droids are designed with the purpose of protecting "objects of value" or to serve as bodyguards. The KX-series were technically classified as battle droids, but the Senate only knew them as sentry droids since it became illegal to manufacture battle droids after the events of the Clone Wars. Many enforcer droids were used in combat against the Rebel Alliance during the Galactic Civil war, such as during the Imperial campaigns on Tatooine and Dandoran.
Not much is known beyond that. Possible in-universe explanations for why they are not seen onscreen during the events of the Original Trilogy could be:
It is worth noting that many of the droids that we do see onscreen (including several mentioned here by other users) in the OT have relatively limited screen time (at least in comparison to droid characters like C-3PO and R2D2), such as:
And there are several other droids, as well as vehicles (both Imperial and Rebel) that have no screen time in the OT films, such as:
This appears to be the most information that can be provided based on the currently-available materials.
According to the story, why are there no Imperial Droids like the K-2SO's in the original trilogy? Why did they all disappear all of a sudden after Rogue 1?
Because they weren't necessary for the plot.
The Star Wars universe is massive with thousands of star systems. We see a small and tiny fraction of this universe in the original trilogy.
We also see a small sampling of other droid types which didn't exist in Rogue 1 - do you need an in-universe reason why any of the other droids within VI, V, and VI were not in Rogue 1?
You're wanting an in-universe explanation for why only a small portion of a massive, massive universe was chosen to be shown in a few two hour snippets. That's not going to exist.
There are Imperial droids. Most notably the MSE droids, the probe droids, and Imperial R type droids.
The droid that runs the droid prison in Jabbas palace was an imperial droid that ran away, according to legends.
There are also supervisor droids—such AV-6R7—as present on the death star and on different imperial ships:
An alternative explanation that isn't backed up by any sort of canon resource and entirely opinion: The Empire simply pulled them. After the events of Rogue One, the Empire could have mentioned how one of the supervisor droids was hacked. This would mean that the entire line might have been compromised, so they could have deactivated the entire line of droids, and replaced them with other droids we see in the series (AV-6R7's series could have been the replacement).
First, since this question implies it’s about K-2SO, I’d like to address K-2SO’s role in Rogue One (2016). Simply put there are tons of droids of all kinds in the Star Wars universe, but few have really been part of the core stories in the films past R2-D2 and C-3PO. K-2SO is different because he (it?) takes on a larger role in the story of Rogue One than any other Imperial droid ever has before.
But like I said, there are tons of droids of all kinds of allegiances in the Star Wars universe: Many of them being Imperial. For example, what about 5D6-RA-7 the RA-7 protocol droid from the original Star Wars (1977)?
5D6-RA-7 seen walking by Han and Luke when they were in Stormtrooper gear escorting Chewbacca to Detention Block AA-23 on the Death Star.
Or how about the E-3PO droid (aka: silver colored 3PO-series droids) who showed up at Cloud City on Bespin in The Empire Strikes Back (1980)? I mean E-3PO model was made specifically for the Empire and that specific unit walked out of a room filled with Imperial Stormtroopers, right?
E-3PO units were a subset of the 3PO-series protocol droid line. Developed for Imperial use, E-3POs possessed the TechSpan I propriety module, which allowed them to interface with a wide variety of networks employed by the Galactic Empire. Their obscure and rare status led to E-3PO units developing arrogant, superior, and haughty attitudes, which would gain them a degree of notoriety throughout the galaxy.
Remember when Chewbacca growled at the Imperial mouse droid on the Death Star in A New Hope? It was when Han and Luke were leading their "prisoner" to the detention center.
It's official name is the MSE-6 Series Repair Droid.
According to the Star-Wars Wikia, the Mouse Droid shows up in all three original films as well as in several Clone Wars episodes and The Force Awakens.
Maybe because the had serious security flaws that allowed them to be reprogrammed by rebels so the empire got rid of them after the horrible incident on Scarif.
K2-SO aside, we only see other KX-series droids in imperial service on-screen inside the base on Scarif. This base was completely destroyed. While there isn't enough information for a concrete answer, the destruction of this base, and the method of such destruction can be used to explain the lack of appearance of many things found in the Rogue One movie that were not in Episode IV. (i.e. Tie Stryker, U-Wing, Many different Troopers, etc).
Also, the official starwars.com databank describes Scarif as "the principal construction facility for the vast Imperial war machine." It is possible that the destruction of a large portion of the planet also destroyed any manufacturing facilities for these droids.
Additionally, there are very few "Imperial Bases" that we visit throughout the original trilogy. All we know is that we have not seen any KX-series droids in service on Star Destroyers, or the Death Star, but that is not a large enough sample to assume that they simply do not exist during the Original Trilogy.
EDIT: For completeness. After rewatching the movie, we also see a KX-Series droid in service inside Jedha City. However, since this location was also completely destroyed, and the one we saw in service was terminated, I think the rest of my explanation still stands.
There is a theory that when they learned what droid was involved in the theft of the plans, the empire probably destroyed the KX droids because of how easy it was to reprogram them.
