The point is that it's the courteous thing to do to ask
Why? Would you call and ask if your wife could wear a pantsuit? Would you call her if you had to wear runners with your suit because your arthritis was acting up? Do you need to call her to see if you could wear your boxers? Would you call and ask if you could wear the purple tie with the pink polka dots?

I'm getting carried away with the examples, but I honestly don't understand why you are giving the bride so much power over your wardrobe. By calling the bride and asking her permission, you are giving her control of something that is none of her business. She may prefer that a guest didn't wear a kilt, but she can't control it, and to impose that much will on someone is actually rude no matter how reasonable her request is.

Now, if you were in the wedding party, immediate family, officiator, performer, etc., I'd concede that you would have a point. But, a guest is under no obligation to wear what his/hostess dictates. The bride may set the tone for the type of dress that you would wear, but not every detail of the clothing of every guest. And even then, you are not obligated to comply.


copied from a wedding etiquette website:

Question Can the bride make it mandatory on the invitation for all male guests to wear a tuxedo?

Answer
What wedding guests choose to wear is ultimately up to the individual guest. Certainly, the bride and groom can decide on a very formal wedding reception (ie. black tie) and while that will strongly encourage male guests to wear dark suits/tuxedos, there is still no guarantee that every single male wedding guest will show up at the reception wearing a tuxedo. This right belongs to each individual male guest.