To put it as neutral as possible, some people in the United States believe that racism and abuse of power go way beyond "a few bad apples" and that "the police" is incapable of being reformed. There have been international examples where the military has replaced the police because the police was deemed unsuitable and incapable of reform.
- Brazil uses military police for peacetime law enforcement. There have been complaints about human rights violations.
- So does Mexico. Again there are complaints about human rights violations.
I'm sure there were some examples where it worked out, but I can't think of any right now.
If there is a problem, it is the culture and not the logo on the uniforms.
Some people disagree that there is a systematic problem. Assuming for the sake of argument that such a problem exists, merging the existing police forces with the military won't resolve it. Dismissing all police and employing the existing infantry for law enforcement might make a difference, but not necessarily for the better -- see the examples above.