Maybe the world just wasn't ready for the PSP Go. It didn't want to let go of discs just yet. Sony have now confirmed that production of PSP Go has stopped, and it should come as no surprise as the company is starting to turn its attention to the successor of PSP - NGP.
The problem with the PSP Go, apart from not being able to play UMD discs, the format which PSP originally were delivered on, was also that the stream of content on PSN was less than optimal. I think it's safe to say that PSP Go owners expected games to come out day and date on PSN, but that was rarely the case. Sony will of course continue to support PSP Go with games from time to time, but there isn't a great deal of PSP titles coming out whether it's on disc or digitally these day anyway.
Bring forth the NGP!