I understand what you mean - it does seem to elevate the killer somehow. I suspect the reason for calling it something different is to distinguish it from the, pardon the expression, more commonplace murders that have existed since the beginning of time that take place out of greed, jealousy, drunken anger or a variety of petty reasons. One murders to gain a wallet. One assassinates to gain a kingdom.

The Webster's definition of "Assassinate" is:

1: to injure or destroy unexpectedly and treacherously
2: to murder (a usually prominent person) by sudden or secret attack often for political reasons