There are practical considerations that prevent this being done instantly. British and EU laws are so intertwined that if you just say we are repealing all the EU crap without making sure what that entails and that there are appropriate British laws either on the books or ready to, could cause chaos.
This is not like turning a light switch on or off. The EU cancer has become so widespread in the British body that it will require careful surgery to get it all out without causing serious harm.
Bttt
Sort of like asking Dr. Ben Carson to separate conjoined twins where one infant is parasitic, and quickly draining the life out of the other.
Complex for sure but not a reason to not go for it - how many times are we told that all the really crappy deals our government has put upon the People are here to stay because “it would be too hard” to set them right?