Because 80x25 uses 4kB of RAM (one byte for character + another byte for attributes) whereas 80x30 would grow beyond 4kB so you would need 8kB. Maybe not a big deal in a VGA card, but everything was standardized on 80x25 from the olden days of MDA/CGA which had little video memory, so a lot of software expected that.
Plus, making the characters shorter would make them also a bit less legible.
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.
Plus, making the characters shorter would make them also a bit less legible.