My understanding is that there are federal grants that are responsible for the militarization of municipal police, largely put in place by corrupt politicians wanting to court the vote of military contractors and their employees in their district. Can't find a source for this ATM though.

edit: here's a pretty lengthy academic article on the matter, see page 1803. https://digitalcommons.law.seattleu.edu/cgi/viewcontent.cgi?...