I think the reason you don't see many freewheels is because there simply aren't many manufacturers making them. Even higher-end BMX wheels use cassette hubs now. Freewheels are more difficult to manufacture, so you won't see as many boutique parts as you will cogs and lockrings. Also, single-speed won't give you mad hipster street-cred, brah.

Shimano, ACS, and White Industries are really the only companies making quality freewheels anymore. I'd avoid anything else.