At the time, it was the cheapest full frame canon DSLR, and I think this has some relevance, A lot of wedding togs have two cameras, so two FF pro bodies, you are talking 8 grand, against 4 grand for 2 5's.
Also, the batteries for 1 series, are a lot heavier than the batteries on non-pro bodies, 5 series batteries are relatively cheap (compared to 1 series) and you can quite easily carry a couple around in your pocket.
At the end of the day, it's probably horses for courses, wedding photography probably doesn't require the AF capabilities or ruggedness of a pro body?