All code WAS written for 2.X, because 3 was either not around yet, under constantly changing flux (before release proper, and to a certain extent for a couple of versions) or the subsequent killer: Not enough libraries that you might depend on, worked in 3.0 yet - as it was written to be deliberately incompatible by removing legacy and old ways of doing things.

IIRC most of the big packages have finally moved over so it'll just become less of an issue over time.