He runs an extremely influential organization that administers a sort of purity pledge that, for most Republican candidates for Congress, is necessary to sign in order to avoid being primaried.
He has decided that making taxes easier to do would make people more okay with tax increases, and so he has decreed that support for the kind of systems other countries use is a violation of this pledge. His excuse is that such a system would effectively slightly increase taxation by preventing people from getting away with taking erroneous deductions. This excuse is obvious, total bullshit.
Grover Norquist wants taxes to be painful because he wants people to dislike government and the concept of taxation in general. Thus, Republicans in Congress must take the same opinion or risk having their purity stamp revoked. Thus, no reforms get passed.
Perhaps this is one area Trump can do some good; certainly he's much less beholden to the traditions that bind the old guard (for better or for worse).