One of the smartest things you can do as a business is to identify where the market is strong and where it is weak. The gaming market right now seems pretty darn strong when it comes to multiplayer content. Part of that is the presence of so many developers and publishers trying to carve out of piece of that pie, but another part is that there is less actual need for more multiplayer games when so many people sink hundreds of hours into just one title. There are people who play the same collection of games year round, only poking their heads up for the annual sequel.
Sony really has no need to jump into that arena. Third party material covers that base and then some.
We can also see Sony employ that philosophy in their release schedule. They know the holiday period will be covered with popular third-party games so they spread their own releases out over the course of the year. I've been saying for a long time that I'm a fan of that strategy.