The crux of the matter is that those Democrats from more conservative areas, who are up for election in 2010, are leery of signing on to a bill that may cause them difficulty during this next election cycle. This most significantly includes the abortion provisions and public option plans in the bill. Add to that some other members that are from left leaning areas, who insist on including these provisions. It seems that it may be very difficult to reconcile these entrenched sides.
Here is interesting background on some of the key players to watch in the current Senate healthcare debate by USA Today: