I've watched his testimony in that video before, and I'm not sure I agree with him. He's complaining about the Tax Reform Act of 1986, which may have had its issues but was hardly a flawed tax reform law. It simplified the tax code considerably and reduced the top income tax rate from 50% to 28%. It was signed into law by Ronald Reagan, by the way. Trump is complaining about it in that video because he owned apartment buildings all over New York City -- and the 1986 tax reform included tax incentives for homeowners that effectively diminished the value of rental housing.
He also says in his testimony that he was invited to testify before Congress by the chairman of that committee. The chairman was Frank Guarini of New Jersey -- a Democrat.
OK, your question was why hasn’t he gotten involved before, which I amply answered, so your reply is another whine that he’s doing it wrong. Here, have a tissue.