re: Why is the the appointment to the Supreme Court a lifetime appointment?

Because the Constitution says so.

Now, the rationale was to put the justices above the political fray, so that they would not be beholding to their masters to keep the bench. However, the problem is that: a) people are living a shitload longer than they used to; and, b) since FDR, federal judicial appointments have become ideological battlegrounds (this really ramped up with the Bork hearings).