SOCOM 4 wasn't actually bad at all (besides the launch online issues that they cleared up).
But it got panned, "wasn't COD!" & "Wasn't SOCOM!"
As for SOCOM, I think i'ts nostalgia more than anything. It's like going back and playing RE1, or MGS1 or Goldeneye. Not many people can do it and still like it, and trying to bring that feel to a current gen game is a lot harder than just porting the gameplay. The gameplay is so dated, that you can't simply port and be done and expect it to sell, as it won't even feel the same.
SOCOM was great for it's time. Today it would need to find the prefect mix of new and a nostalgic feel to still be relevant. That is impossibly hard to do IMO.