Mostly because they aren't user-inputtable. Yes you can't realistically hand-edit a 3gb .csv either, but csvs don't usually start out that large.

I was going to add a sentence to my comment... They were easy to input via control-caret, etc., until various programs repurposed those combinations. I think a bigger problem is that there is no standard way to display them.

(BTW, if inputtability mattered at all, Unicode wouldn't exist. ;-)

I'm not sure what you mean by your last sentence. Sure, you might have to work a bit to be able to change your locale to Klingon, and input that -- but for something for which unicode is actually useful, like Japanese, there's been input solutions for a long time (predating unicode, as you need a wide character set for such languages).

And unicode is of course easy to use with "less exotic" languages like Norwegian that can't be represented in basic 127-bit ascii -- via a simple keyboard map.