David Joulfaian has written a lengthy manuscript on the history and background of the Federal Estate Tax. Its a small portion of US revenue, however you will need to read it to appropriate why its not going anywhere, at least not for any substantial amount of time.
Some form of inheritance tax has been prevalent through much of recorded history. Like all taxes, it has been controversial and there have been many changes in the way its been calculated. There is a comprehensive review of the evolution of the estate tax in the paper itself, which you can download here.