I think it has to do with lots of people needing to feel superior than others. In order to feel good about themselves, they need to be able to look down on folks that are not like themselves. And many politicians, especially populist ones on either side are more than happy to fuel that sentiment and poor gasoline in the fire for selfish reasons. And with looking down on some groups there arises a perceived need to limit what that group is allowed to do.
Historically it has changed what that group is that is being looked down upon. Women, left-handed folks, Jews, Blacks, Asians, Latinos, homosexuals and now us, at some point in time they all shared the same treatment. Once the focus shifts away from us (and it will at some point), there will be another group that is in the center of being looked down upon and having their rights revoked.