Price and games. The ps3 has no JRPGs aside from Folklore and Enchanted arms, the first of which is more Action oriented then most RPGs, and the latter of which just sucked. As more JRPGs, and some other popular game franchises in Japan start coming out sales will pick up.
HSG5 and DW6 did their parts, but those two games alone were not enough to keep the ps3's sales afloat. MGS4 will help as well, but it really needs something like GT5, or the even bigger FFXIII to really push consoles. Big games like this, coupled with a steady release of games like WKC and ES in between will help increase weekly sales.
The only problem is that games like FFXIII, White Knight Chronicles, Valkyria Chronicles, Eternal Sonata, etc. are still a long ways off.