I think yet another reason for the uniform was to have Michael stand out that much more from the rest of the family and guests. Not only do he and Kay (in her bright red dress and big floppy hat) make an entrance together, they also visibly stand apart from the other Corleones...both individually and as a couple.

And of course MP and FFC deserve an aweful lot of credit for details/nuances like this one. It is one of the many intracacies that weave the story.

Apple