Some possible reasons BCV don't Take Over The World(tm):
1) Perhaps there are limits to how fast an individual BCV can create others of its kind. Perhaps it takes something out of the Master (in the form of consequences) for each new BCV created. Depending on the consequence, this would enforce a limit of progeny creation (especially if the consequence was a mental one, rather than physical).
2) Perhaps there are limits as to how fast a newly created progeny 'matures' to the point it can create further progeny. If this delay is large enough, then it reduces the exponential nature of the problem, especially when combined with #1 above.
3) There are almost certainly risk-based limitation. Say a BCV started creating a new army, 100 new BCVs per night. How long would it take for the mortals (let alone the White Council or other supernatural entities) to notice the scores of daily missing persons reports?
4) There are likely control-based problems. How fast can a master BCV establish long-lasting control over newly-created progeny? Even if he creates 100 new BCVs in a night, and even if they are initially obediant, how long until that changes, and the 100 progeny start thinking that maybe one of them would be a better boss? I imagine that a master BCV might only create however many progeny it feels it can handily take on in a coup attempt.
5) New BCVs are almost certainly weaker than full BCVs. Part of this might be because they were 'taken out' in order to be created in the first place. So even the master's future lieutenants (ie, the ones that Fate has chosen to give multiple consequence slots to) will have those consequences filled until they healare repaired. Maybe this takes a while.
Some ideas to play with; do with them as you will.