Cults are not just "baby religions" or ones that haven't "taken off" yet. There is specific criteria required to be classed as a religious cult, a checklist with a certain number of markers required, governments do monitor these things as no one wants a doomsday cult massacre on their hands, some are dangerous, others simply deluded. It typically involves a revered cult leader although it is not an ironclad requirement, there is an element of isolation "us v the world", there is typically dependence by the followers on the cult either financially as they it manages their money, or through physical isolation as in a closed in compound (this is the scary one for potential doomsday cults), or some other emotional dependence such as separation from family if one were to leave as the cult is more important than family, JWs sure hit people hard with this one, if you leave you must be shunned.
I would expect that there are two ways cults start; as splinter organisations of existing religions seeking greater restrictions/control, or via a dynamic convincing leader starting their own movement.
As to why people join/why they are successful, well that depends entirely on what the cult is offering, the people who survived Jonestown talk about the sense of community, feeling as though they were living as one big family without fear or unmet needs until the Flavor aid, they were looking for utopia, I think this is what religion in general can offer to people who feel lonely and are searching for a place to belong, the more extreme the offer, the more the people searching are looking for that one simple answer to sort out everything in their life that isn't working. The outward appearance can be one of respectable community, fractures and hypocrisy are hard to see until you are within it, and when you are within it, you are ripe for manipulation. And of course: The simplest way get members is for existing members to have children, indoctrination is easy when you get em from birth, the successful cults therefore need to be sustainable in future generations.