I think its pretty obvious why.

- Its not a very good game. Zipper has failed to deliver much quality this gen.

- PSN going down right when it came out killed a lot of online only players purchases.

- 2 much better games came out at the same time, Mortal Kombat and Portal 2.

People, it's very simple, the two reasons it failed are painfully obvious:

- It took everything that made SOCOM "SOCOM" and threw them away, in favor of jumping on the COD bandwagon, and it failed miserably in that aspect.

- The PSN outage didn't help.

A lesser reason:

- Confrontation didn't exactly leave the best impression on future SOCOM games. Mind you, even Confrontation was more faithful to the SOCOM formula than 4 was.

game is not bad! but its not like the other socom games...they changed to much

Don't forget the horrific advertisements. The game is not visually striking at all and the whole advert is gameplay footage of fighting. It looks so generic and old I don't know how anyone would have thought it would help sell the game. Complete waste of resources. Someone should have lost their job for that.

They did lose jobs. They were recently hit with layoffs.

The game sucks and hardcore fans couldn't get online to play. SOCOM 4 is now old news. The end.

Was an average game.

As if the hardcore fans wanted to play it.

Meh, I still plan on buying it. Me and my brother need another game to play together and we're almost through the coop for Killzone 3.

I bought it and played it as I had nothing else to buy at the time.
It was my first Socom game - I don't play shooters online so was only interested in the single player campaign.
It is a totally generic 3rd person shooter with mediocre story and gameplay and the reviews reflected this.
It sold as much as it deserved.