I think it's because we havent bothered to put a censer button on all our cell phones. I bet if we did, conversations would be a whole lot more fun, and also, help promote hand-voice coordination.
Like all things, used sparingly, curse words or in Spockian terms (as in Captain) "Colourful Metaphores",
can retain their effectiveness. Problem is, they are overused.
To quote DC Curry, " Back in the day if you hollered at a girl and you called her B---h, she knew exactly what you thoughy of her and how you felt about her. Nowadays, if you holler at a girl B---h, she's trying to figure out where she knows you from"
I think part of the problem is that we have not as a whole been taught to argue or discourse without anger or overt passion. Think about what we are being shown on TV all the time by our politicians and celebrities. The ones that get quoted the most often are the ones who yell, even nonsensically, at the audience.
So, the only other way we see perhaps to enhance that is to use explicatives, even in a grotesque nature. I'm not justifying this behavior at all, simply stating a reason why.
BTW, the youtube link is not in any way meant to be political, but it was simply the best example I could think of.