the same reasons why everybody else is not switching to bitcoin. the government is one of the biggest reasons, they are never going to let people switch to something they can't control. besides bitcoin is not the stable money you would like to use as your daily cash.
As far as I know, the Venezuelan govt is not restricting BTC. And believe me, BTC is much more stronger and more stable than VEF.
The problem is most people don't have any extra to try and invest in BTC and there just aren't merchants over there like here. For the well to do in Venezuela, BTC is great and I've been telling my family about it. Some are still doing okay over there and can afford to save in BTC.
It's just U.S dollars is easier. People like holding cash. Everyone knows the black market rate in dollars and wants them. Bitcoin is just too new and grew too late to help Venezuela. Especially cause the power goes out constantly.