Or it could be that it's much easier for them to beta test the PC version themselves internally, so they have no need for asking other people for help.
Yea, from what I've heard so far, they're only testing the PS3 version because it still has some issues.
They also invite European players only, because they are located in the UK and they can sew people if they break the beta test agreement (They are not allowed to leak ANY information)
Please don't ask for a beta on all platforms, if there comes one, it will be for testing and reporting issues, not to play the game before it's released.