Why do the leaf of my Wisteria grow to quick and cover the flowers which are in full bloom

That's Wisteria for you, cant change habbits of a life time!!

It is quite natural for that to happen. If you want to avoid it, though, you should learn how to prune it properly - it's quite complicated, but done correctly, the leaves will start to appear after the flowers are more or less done. If I were you, I'd just be grateful its flowering at all! and cut off the leaves which are blocking view of the flowers.

Yes, pruning would help. In autumn cut off all laterals to 6 inches unless you want some for training to extend the plant. Then in winter cut the shoots down to 2 or 3 buds. There will be less leaves as there are less leaf buds. Be quite ruthless with the long shoots as this plant soon runs away with you!