I think there are several things, but ultimately, conservatives finally felt this was a culture war outrage they could stand up against and win.

Not that beer, or Bud Light in particular, is representative of conservatives. But there is a specific class of people (working class, lower income, etc) who tend to drink Bud Light. In the US at least, that class is often conservative or moderate with traditional values, so it struck a nerve. They saw right through the propaganda and wanted to make their feelings about it known.

If Bud Light had gone with a softer approach to inclusiveness - maybe a gay male spokesman who lives a more traditional lifestyle with his husband and raising children - I don't think the reaction would have been anything like this. Instead an uber-woke marketing woman decided to completely ignore the audience of people who are receptive to Bud Light. She brought in not only an offensive caricature of women, but also one who is not a "good gay".

Mulvaney's personality is the campy/dramatic type that bullies perceive as weak, and he was likely beat up and called all sorts of names for being gay growing up. And now this "queer" (slur, not alphabet-soup letter) is the face of a brand that is practically synonymous with their lifestyle. There is an undercurrent of homophobia in the dislike and contempt, for sure.