Because of two very important factors.
1. As with all subscription services, the first few months offerings are GREAT. Thats how they draw you in on their offer. Then after a while they start cutting back on the content. The games aren't so good, it becomes the poor-selling PSN games which they start to give away. Its the same with the Sky service we have in my country, at first the content was excellent but then they started adding advertisements and things like that and the quality of the movies were often better on over the air TV sans the advertisements which I timeshift.
2. If something on PSN is worth having, its worth buying. What PSN does is make you pay twice for the really good content, first for the content and secondly for the subscription, or offer you content for free which you never thought was worth the purchase price to begin with. So if you have PSN you either don't buy great content and hope it comes on PSN or you buy the great content and then get pissed off when it gets offered for free at some later date.
Furthermore the more they sucker in with the service, the more they will start to offer services as PSN only. One PSN subscription is just one more vote for them to turn PSN into Xbox Live.