Why has so little technological process been made in the 250 years between the time Kovacs was stored in digital form (100 years from now) and downloaded into a sleeve formerly inhabited by Ryker (350 years from now)?
The best answer to your question is that you have forgotten what you were told about Takeshi's mindset. Takeshi is specifically trained to integrate into new worlds and settings at an extremely rapid pace so that he doesn't stick out like a sore thumb. His apparent lack of surprise at technological advancements, and near complete absence of mentioning them, should not be taken as evidence of the lack of such advancements, but as evidence that he's already adapted.
The series is also kind of subtle. It generally doesn't sit there hammering into you things like technology changes or go into lengthy soliloquies about the ills of society unless it serves a specific purpose for the immediate plot. You have to pick these things up for yourself as you go along.
That said, advancements have occurred.
Poe has to specifically tell Takeshi to use his DNI rather than the antique TV. He uses it whenever possible from then on, so it's not like he had some philosophical stance against using it; he just didn't really know about it.
There are a couple of other instances of (implicit) technological advances:
Artificial limbs are undergoing progressive advancements, evinced by how the doctor goes through some lengths to sell the top-of-the-line model to Takeshi for Ortega's replacement.
Takeshi did seem rather surprised at the Aerie, suggesting such ridiculous structures are a new thing to him. It would take rather advanced building methods to build such a thing and keep it stable. Not to mention that there should be severe winds, cold, and oxygen deprivation up there that no one has any problems with even in an open air environment.
and...while I was typing some of this I thought of another one, but it seems to have skipped my mind.
For other matters, an undercurrent of the setting is that the rich live forever and have everything and can do whatever they want, with everyone else being their pawns and slaves. Most of the populace is living in a stagnant world (if not one in decline) because this serves the goals of the Meths. It's easier to keep the rabble in check if the rabble don't have any power or things to threaten you with. Takeshi spends most of his time among the poor, so we see their world most of all. Most impetus for technological advancement then gets removed. Only the rich matter, death can be little more than a minor annoyance, and some metaphorical bread and games and (sometimes literal) opiates for the masses takes care of the rest.