Because it's embarrassing and humiliating to go to your best friend's wedding without your SO for such a ridiculous and childish reason? OP RSVP'd, she knew the dress code on the invitation - personally, I would have been a little miffed because not all women want to wear dresses, but that's the terms you agree to when you RSVP.

OP is the AH because,

1) She should have reached out to her boyfriend's best friend (so presumably they have their own relationship as she likely sees him often) and asked for clarification on the dress rule or asked for an exception months in advance, rather than 2hrs to the wedding.

2) She shouldn't have RSVP'd and then not attended - in C19 times, when weddings probably have guest limits, she's wasted a seat that another friend or family member could have used to attend the union. & she's wasted the couples' money because they paid for food for her.

If I was her boyfriend, I too would have made an excuse instead of having to explain that my SO was uncooperative and unwilling to follow the dress code.

I get where you're coming from OP, but I don't think this was the hill to die on. YTA.