Because fame and ego. Assange is a weird-looking socially awkward epsilon-male, but make him the figurehead of anti-Americanism and apparently a bunch of Swedish leftist chicks want to bang him. At least once. As long as he puts on a condom.

In all seriousness, if you find yourself asking "Why did this man choose fame, glory and groupies over the good of the cause he claims to espouse" then you don't know much about human psychology.