And maybe cost also? Not sure, but back in the days maybe 4kB more would have a noticeable (albeit I do not think extrem) effect on final price.

Not in the days when these BIOSes were written, but in the early eighties, yes; the original IBM MDA card had only 4kB memory (enough for just one screen of text) because memory was expensive. So sticking to 80x25 was kind of important back then.