My guess is a lack of political will to do so. It's hard enough to get new political agendas passed in some cases without the added burden of trying to introduce something to take care of a law that is currently unenforceable, and would also appear to be unenforceable post removal of protection from the consequences of that law.

My second guess, and it could be mixed with my first, is that unless you have a really solid party majority you could be inviting a voter response for tackling a wedge issue and it might not be perceived as worth it if the law is unenforceable anyway.