Back in the day, we saw a well-built guy and thought he was a physical specimen that was larger than life. Nowadays, we see a well-built guy and think "Yeah, that guy's on steroids." Even if the guy is naturally built (if that's the proper word for it), people will immediately assume they're on the juice. And if drug tests turn out positive, people will cry foul and assume it's a cover-up.
So Brock Lesnar failed a drug test. Whatever happens to Lesnar now has yet to be determined, but I'm fairly certain that no matter what happens, "Steroid City, Bitch" is the three words I'd use to describe Brock in a nutshell. And even if he comes up clean as a whistle, it's sticking because that's all I got.
All things considered, it's safe to say that this puts a damper on any UFC aspirations Brock might have. And as far as WWE is concerned... like they fucking care what happens.