Maybe because some guys like the older versions?
I dunno, but, from what I've seen, there are mods for the older versions which are awesome. For example: eggmanway5, CBFDRB2, SRB2 DX, SRB2 Unleashed, SRB2Riders, SRB2Kart, XSRB2 and so on.
But, there are some guys playing the 2.1 version, so...