Why would you want to play an outdated version of Tekken?
Cause I like Tekken?
And if the best portable Tekken I can get at the moment is T6, I'll take it. I'd have bought the Tekken 6/Soul Calibur PSP pack for $15 if SCEA lowered the price to be equivalent to the SCEE store already, but they haven't so I haven't.
Free is fine by me.
I'd assume it's the same reason anyone would want to play the Powerstone Collection or SF3AlphaMax or anything like that. :P
When Scamco decides to port TTT2 to Vita, I'll gladly buy it to replace T6 for my portable Tekken fix.