Well, as the name suggests jRPG are made in Japan, mainly for japanese audiences. Yes, the genre has seen successful games in other markets but Japan remains the center of it. Japanese developers have been slow developing not just jRPGs, but anything for HD consoles. Since the 360 is a non-factor in Japan, it's unlikely japanese RPG developers will care to go multiplat just so they can sell a few thousand more units outside Japan. When a company like Mistwalker can't get a big budget game like LO to break the measly 200K barrier in Japan, developers know they're better off developing for the PS3.
However, the PS3 is still waiting for those titles and its installed base is so small in Japan is a good reason for concern and question whether or not is worth the hard work of putting together a true "next-gen" RPG that can attract the gone-casual majority. It's kind of a vicious circle, this current situation for the PS3 in Japan, but the fact remains japanese gamers still love RPGs and if a recent release like Valkyrie of the Battlefield is any indication the genre can sell on PS3, we can be sure we'll see more developers bringing new exciting projects to the console.