Dear DrAgony Auntie - Do I Smell?

Maybe your colleagues have all worked together as a team for longer than you have? Maybe they are all of the same age group (give or take about 3 years) than you? maybe the one responsible for invites does not personally like you and so is biased with invitations?

I would not take this personally because, as you mention, you are not the only one to be excluded so (providing you actually like your other colleagues who also have received no invites) why don't you arrange a social night at a pub for all colleagues also neglected with the social join up for a pint type of "do"?

Pretty mean of the form as this type of neglect does hurt but make something out of it and laugh about it and arrange your own pub night with others also shunned and I am sure you all would end up having much more fun.

Well...... it's possibly a combiniation of all three of the reasons you gave for them not to invite you.
So I would suggest perhaps you change your soap, learn a few new dance moves and perhaps try a little harder with your job and see if the situation improves next year.
If it doesn't ..... tell them where to stick some soap, tell them they can stick their job and I also saw some rather interesting instructions on an anti-perspirant can recently which read - remove top and push up bottom - maybe they could learn something from this.
Hope this helps