This is primarily for two reasons. Firstly because it is only so long before a game franchise becomes repetitive and unorginal. Also most game devolpers only have enough time to make 3 games a generation therefore they amke a new franchise very generation just like naughty dog.
The main reason for him leaving is to pretty much stop marketing for the psp in general.As sony plans on releasing a new handheld in the near future it would be more beneficial for forwarding potential consumers to sony's new handheld because that is there new focus. It's a trend in the indistry when ever a new generation releases the company generally stops supporting and marketing the older generations and models.
Companys are always creating prototypes. The only thing we need to wait for is the approiate time to launch a console. for example right now it would not be because it is currently the climax of each consoles lifespan.