1. Tax law is passed.
2. Really smart CPAs and Tax Lawyers figure out ways to legally avoid paying taxes.
3. Tax people issue regulations intended to close those loopholes.
4. Administrations change and they want to win votes and tweak the economy so they lower some taxes and add tax breaks.
5. New administration comes in and selectively reverses previous administration's program because of politics.
6. If the tax is international tax, then add really smart tax lawyer doing multi-jurisdictional tricks to lower taxes (e.g Double Irish With a Dutch Sandwich[1]) , governments trying to encourage multi-nationals to do business there by adding incentives to their tax code, the OECD coming in and trying to standardize tax stuff across jurisdictions to prevent this, individual pairs of countries doing tax treaties with each other, etc.
[1] https://www.investopedia.com/terms/d/double-irish-with-a-dut...