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.

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f1shy 3 days ago | parent [-]

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.

	▲	michalpleban 3 days ago \| parent [-]
		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.