score:1
It seems to me that the answer is, in a word, politics. While it's true that Jesus himself was Jewish, Christianity really came into being later, among pagan Romans. For various reasons, it was politically expedient to set it as a new religion distinct from the Romans. Late still, it was politically astute to take a step away from Judaism.
Similarly, Mohammed was spreading Islam around pagans for whom monotheism was a new concept. It was politically useful to have a mythology to point to.
On the other hand, Mormons don't want to distance themselves from Christianity, so that the difference seems manageable for new converts.
Again, the big Christian schisms, like the protests of Martin Luther, can be seen as new religions. However, politically it would have been a bad idea at the time to completely deny the church, so Protestantism became a sect.