Why wasn't the Room of Requirement used more often?

Question

The Room of Requirement is extremely useful, but Harry barely uses it. Sure the DA uses it, and so does Harry to hide his Potions book, but for half of the fifth book, and the sixth, whenever he needed anything (minus food) the room would bring it to them. So if he was worried about his O.W.Ls, then he could tell the room to help him out. Or if he wanted a secret passage to Grimmauld Place, he could use the room, instead of sneaking into Umbridge's office.

This is not asking what the room's limits are, but asking why Harry doesn't frequently use it.

Answer 1

Out of universe, any – especially magical – "Get Out of Jail Free" card (a sort of Deus ex Machina) can be overused as a plot device. If Rowling used it to solve every small problem, the books would suffer, and the characters wouldn't "develop" to overcome obstacles using their own resources.

If you accept the out-of-universe justification for limited use, then in universe – despite the OP not wanting to talk of the room's limits – then we have to assume that it does have limits, and those limits are (reasonably) high: you cannot use it just because you fancy an ice-cream!

An answer to the no-longer directly linked question What are the limits of the Room of Requirement? includes (emphasis mine):

[The Room of Requirement] is a secret room within Hogwarts Castle, that only appears when a person is in great need of it.

This is echoed on the Room of Requirement page on WikiBooks (again, emphasis mine):

Dobby the house elf tells him [Harry Potter] about this room, describing it as, "A room that a person can only enter when they have real need of it. Sometimes it is there and sometimes it is not, but when it appears, it is always equipped for the seeker's needs."

I cannot remember (but it has been a while since I read the books) of any instance of any of the main characters trying to gain access to the room and failing, so it's unclear whether they tried and failed to use the room for "lesser" needs, or they simply realised that it would only work for "great need", and never tried. However, I seem to remember a scene (at least from the films) where someone (possibly Filch, possibly Umbridge) was trying to follow Harry in to the room and it failing to open. If so, then this could indicate that it wasn't just personal need that mattered (the need to catch Harry didn't override Harry's need to not be caught).

In summary: the room does have limits, fairly high ones. Either the main characters knew these limits and didn't try to push the boundaries, or any failed attempts weren't included in the narrative.

Answer 2

Not everybody knew about it

We know that some of the teachers knew about it:

"The Room of Requirement," repeated Harry. "Were you trying to get in there?" "I - well - I didn't know students knew about -"

This quote also implies that other members of the staff knew about it seeing as she specifically refers to the students. We also can be sure that Dumbledore knew about it:

"Oh i would never dream of assuming I know all Hogwarts' secrets, Igor" said Dumbledore amicably. "Only this morning, for instance, I took a wrong turning on the way to the bathroom and found myself in a beautifully proportioned room i had never seen before, containing a really magnificent collection of chamber pots. When i went back to investigate more closely, I discovered that the room had vanished. But i must keep an eye out for it. Possibly it is only accessible at five-thirty in the morning. Or it may only appear at the quarter moon - or when the seeker has an exceptionally full bladder."

Later when Dobby tells Harry about the room of requirement for the D.A Harry makes the connection:

"This room isn't just some mad idea of Dobby's; Dumbledore knows about it too, he mentioned it to me at the Yule Ball."

The only students who ever used it were Harry, Ron, Hermione, The rest of the D.A and Draco Malfoy until Harry's seventh year when the "Good Guys" hide from the rest of the school:

"Where are we?" "Room of Requirement, of Course!"

Another possible reason is: People only ever used it in dire/near hopeless circumstances I'll go through all the times it was used: First, when Dumbledore needs to use the restroom (this is more of an everyday type of desperation) he gets lost and must have desperately wandered around until the Room of Requirement presented itself. Next, Harry uses it when he is in dire need of a place to hold the D.A sessions. Finally, (that i can recall) he uses the room to hide the Half-Blood Prince's old potions book (this was also somewhat desperate since it contained a spell that lacerated the targets skin). In conclusion i would like to assume that the people who knew of it wouldn't use it for anything that wasn't hopeless or very needed. Another contributor to the fact that it wasn't used on for example; a daily basis is that the students/teachers always had full schedules, so it would be difficult to fit in time for using it for trivial things, also whenever people entered the room they seemed to spend lengthy amounts of time, so if they got absorbed in whatever they were doing in there they might miss a class. Finally, it might be that people were too lazy to actually go to the room for whatever they needed instead of just making do.