... because third parties and great diversity of games IS PlayStation. They still have exclusive bits of content, exclusive first party games and console exclusive or time limited third party games coming over the next year, but it is important that special new conferences like PlayStation experience keep pushing not what gaming on PlayStation is about and has been about since the PS1. Sony has always used its first party developers to push boundaries and increase the system appeals rather than pure blockbusters meant to mainly drive revenue in and of themselves although they managed to get that too .
There is GDC, there is E3, there is PGW... you cannot expect lots of super huge AAA exclusive titles at each conference all in the span of a year. Adding PS Experience and giving third parties the spotlight, especially giving plenty of time to smaller titles and/or smaller publishers and focus on game experience diversity is a stroke of genius IMHO.
Shu just confirmed he views PSX as a lesser show than PGW, so I guess that explains the lack of a focus on first party games.