You can, in some sense, blame the passage of the estate tax amendment on Frank Blethen. Blethen is the majority owner of the Seattle Times and one of the prime members of a group that has been spending millions to save billions, or at least try to save billions. And he uses his paper as a bully pulpit for this issue; almost any Washington State Democrat whose district is at all competitive tends to support estate tax repeal (or in this case, steep cuts in the tax), if only to avoid going twelve rounds with the Times editorial board and risking negative coverage. What's more, the state party remains traumatized by 1994, when Washington was Ground Zero for the Gingrich revolution. Of course, in addition to the national party falling apart, the state's governor appeared at the University and made noises about legalizing marijuana, vetoed bill banning the sale of violent video games to minors, and proposed his own statewide version of Clintoncare. Needless to say, the current governor isn't going to make those mistakes. Thus while from the outside, Maria Cantwell and Patty Murray appear extremely safe, in their own minds they're just one false step away from crushing defeat. Hence their ridiculous votes on the Lincoln-Kyl amendment. Had both Washington Senators voted "nay", the amendment would have failed.
Put this one on the list of "things to be killed in conference".