The key was also simplicity. Other proxies had popups and redirects everywhere, audio banners, couldn't be trusted to not give you a virus. HMA just worked. It's the same reason Imgur made a killing.
And sadly I get redirected to some sinoadv scam sometimes when I visit imgur on my phone or tablet (adblocked them on my PC until they fix this). I'm sure they're much more careful when selling ad space in the US, but they're doing a horrible job outside the US.