You probably wear suits that fit and the confidence probably shows through. Not to stereotype, but I suspect a large number of developers have one or two suits they wear for job interviews, weddings and funerals, and they bought them long enough ago that they are too loose or too snug by now, and consequently feel uncomfortable when wearing them. At least this used to be me.

Personally, I feel uncomfortable in any suit, not because of size or fit, but because I must be wary not to make it dirty at all, otherwise cleaning is a pita. Say you want to go for lunch with coworkers and someone orders spaghetti/pasta. Great ... I rather wear something simple, so there is no problem, if something unfortunate happens.