Horford is absolutely killing it in Atlanta this season... unfortunately (or furtunately rather), he plays alongside JJ, Josh Smith, Bibby, etc, and a stacked ATL team - so he isnt as recognizable as other centers in the east.
On the flip side, when you think of New Jersey, you automatically think of Devin Harris and Brook Lopez. he is more recognizable and is one of the stars on NJ. not a role player like Horford is in ATL. and that makes a HUGE difference when it comes down to allstar fan votes as well as some coaches and media votes.
that being said, I think Lopez will be the next young big man to become an allstar.