It used to be that the only men who wore tights were hardcore cyclists, poofters in productions of Robin Hood and poofters who did ballet. If you were a normal bloke and were doing a physical activity in the cold your legs were covered by trackydacks. Were a man to turn up to footy training in 1995 wearing tights he would suffer a world of humiliation and pain.
Times however have changed. Flick on the news and you'll see whole teams of young footy players preparing for their big match with only a thin layer of fabric separating their balls and the rest of the world. Cleverly marketed as "compression wear" these tights are supposed to magically prevent muscle damage or something. I don't believe that is the reason for their success though.
Firstly, I believe many men have a strong urge to put their cock on display and now that budgie smugglers are frowned upon athletic tights have proven an excellent outlet.
Perhaps more important than this though is the fact that athletes want to differentiate themselves from junkies. Trackies are a junkie uniform and when wearing them people pretty much assume that you are going to steal from them. With tights on however there is nothing to hide.