Because other sites like Google and Facebook aren't a platform for which criminals can exchange illegal goods and shop for illegal things and buy them.
Google and Facebook sure as hell are platforms where people do tons and TONS of illegal shit.
Plus, no offense, everyone who used Silk Road knew the consequences of how shady the shit was when the site first came on-line, there wasn't any allusion to the fact that a transaction might not even make it to the seller and might be seized. Just because he's the owner of the wesbite doesn't mean he can claim willful negligence for the things that happen on his site.
In that case, I also know how shady the shit is with banks. Hard to find any bigger and worse criminal organizations than Goldman Fucking Sachs and the likes (drug cartels are child's play in comparison). Where does that put me as a user of fiat money?