Because those hairs that have regrown on Minoxidil are "Minoxidil-dependant".
DHT made them fall out ("miniaturize") a long time ago; which is why you are using minoxidil.
Just because finasteride is inhibiting DHT doesn't mean that those hair follicles that are particularily susceptable to DHT can flourish, even in a DHT reduced environment (otherwise all we would need is Finasteride).
minoxidil is great, because it allows even these DHT-susceptable hairs to potentially flourish. But remove the minoxidil, and its back to square one.
I guess a good analogy would be that Finasteride merely shields the hair that you have now from further hair loss. When you add Minoxodil, you're just supercharging your hair to grow faster than you can lose it, but the minute you lose that super charging, your hair will go back to the "shield-line" (eg; Where it was when you started taking Finasteride)
Either way, I really need to get on minoxidil.