Well, there's been a bit of discussion on that in various sexualization threads. Some of it is that and some of it is simply preference. When asked about it, Tom Brevroot said the character couldn't be taken seriously in the speedo, which is just BS, IMO. Namor was taken at his most serious as a character by Marvel in the 40s - 70sl, when he wore the speedo, as opposed to now, when he's in the black suit. He also sold much better and was far more popular.
OTOH, there's a good reason for the black suit in All New Invaders. James Robinson and Steve Pugh haven't said as much, but it seems clear to me, that they are using the new black suit to differentiate between Namor's adventures with the Invaders in the present, from his speedo wearing days in the past, that they flashback to fairly often.
I tend to agree with what you said, it makes him unique amongst the superhero set, and it fits the character. But even amongst Namor fans, there's folks that like the full body black costume and those that don't. I prefer the green swimming trunks, myself, obviously.