Here's a long list why.
1. The 2nd amendment. Shall not be infringed is straight forward.
2. Registration leads to confiscation. California, New York City, Chicago, and Washington DC prove that.
3. Taxes and fees. If guns are registered, if oftentimes costs money to do so. That's on a county level here(Although Livingston is nice and didn't charge me).
4. It doesn't work, nor help. Michigan isn't a low crime paradise, and we have registed pistols here since the 1920's. That was also due to special interest known as the Ku Klux Klan. They pressured the legislature when a black man used a gun in self defense against a Klan Member.
It also leads to high violent crime rates because criminals no longer need to fear their victims.