It's very obvious why they grow (or rather, why they did).
Wolves in sheeps clothing - the effort they put into the outer appearance.
- They appear radical to devotion, which is definitely attractive, though the truth is that it is not a 'devotion' generally, but rather a bargain or an enslavement.
- They cover up bad publicity.
- They eject any 'problem' people and label them.
- They focus blame on certain societal groups (that's a big one).
- They play on the fear that is generated in people by their own sinfulness.
- They make spurious claims in the theological area, which bamboozles those unfamiliar to it.
- They appear 'nice'.
- They say they love you but don't (the spiritual fornicator)
I could go on all night, but the main thing is "appearance".
Bang