Just seems wrong to have it beat by some wild card aided 5 of a kind. After all, you only get a very very small number of natural royal flushes in your life. People don't want the Royal Flush to be beatable.
I think that is what we have a case of here. I guess it just preserved the feel of the game for some people. Therefore, as a compromise, there was a brief period in poker history during which a straight flush beat 4 of a kind unless that 4 of a kind was AAAA in which case quad As wins. However, the old time players weren't too keen on the new-fangled way the kids were playing the game and it just didn't seem right if AAAA was not the best hand. The game evolved and straights, flushes, and straight flushes became widely accepted. This was because the straight and the flush (hence the straight flush) had not yet been invented. A long time ago, the best hand possible in poker was AAAAx or KKKKA.
