Wait, they stopped requiring it or they just shut down the PS2/Wii games entirely?
It still would've needed SCEE to updated Ad-hoc Party. For whatever reason, Sony's western branches refused to lift a finger to support MH on PS3.
The PS2 games have been shut down for a while now. I think they stopped support sometime last year in Japan for them, IIRC.
As for SCEA not wanting to upgrade Ad-Hoc Party, I think it was more of a money issue, since they'd also would have had to update their infrastructure to support the upgrade.
Just to do all that for one game that might not be that huge of a seller probably didn't seem worth the trouble.