Hi  - Yes I do really believe there is a rational explanation for why these types of cults exist in the US but are unknown elsewhere. 
In what universe is it "offensive" to state the simple objective fact that such cults are crackpot? Are you seriously proposing that David Koresh and his followers were perfectly rational and sane? What makes you think Jim Jones was no different from any mainstream Methodist/Presbyterian/Baptist/Lutheran minister? Do you seriously think closed armed cult compounds could exist in Sweden or Italy? Do you believe that polygamist churches could be established in England or Germany? Are you claiming it is possible for the Westboro Baptist Church to exist in France? Yet all those do currently or have existed in the US - there simply must be a reason why.
My question is no different than asking for reasons for the rise of anti-western Islamic radicalism.