In many cultures, especially in the west, it's inappropriate to dress in black when going to a wedding. Black is the color of mourning, most appropriate for funerals, and most inappropriate for weddings, which should be a joyous and festive occasion.
Not so in Finland. For some strange reason in Finland 99% of men dress in black for all formal ceremonies, including funerals and weddings. It might be hard for a non-Finn to imagine it, but I swear that the typical Finnish wedding looks like a funeral, at least if you look at the male guests. It's not a hard rule, just a de-facto custom. And yes, it looks ridiculous for such a festive occasion.
Most Finns will joke about it when pointed out, but seriously, I really can't understand the reason for this.