I think differentiation is probably more a symptom than a cause. Cults are lead by an authoritarian regime that typically seeks to inflate their sense of importance and control their followers. How are they to show that they're god's divine channel if not by changing doctrine somewhat? Religions that aren't cults probably have more mainstream views just because they don't seek to control their followers so there's less incentive to going out and inventing new doctrine - they just follow whatever is the norm and don't place nearly as much importance on it.
It also seems that you're unnecessarily restricting yourself to cults based on christianity. There are cults that don't have any doctrine that relates to christianity (therapy cults, pyramid/investing scheme cults, etc) but it's not that fact that makes them a cult because there are other groups that have a similar format/goal but are healthy (healthy therapy groups, normal investing clubs, etc).
Furthermore when a cult becomes mainstream and begins to shed some of its control (the early catholic church and even early christianity seem to have been cults in their day) the doctrine necessarily has to be viewed a little less stringently because the only way to prevent differences of opinion on doctrine is to exert cult-like control on followers. As the control fades the doctrine becomes fuzzier and drifts to more mainstream teachings that most everyone can agree on and avoids ones that are more contentious.