I wondered why they do it back then, and wondering about Twitter now: when trying to access those from Russia, if you use Tor, it's easy to access the website on its regular address via an exit node. Of course an .onion wouldn't harm, but it doesn't make much difference. On the other hand, the government tries to block Tor in Russia since the last year, the bridges that used to work don't work anymore, and even if you obtain new ones to which you manage to connect, somehow Tor still fails to complete a connection to the network (I didn't investigate further yet). So a regular mirror is likely to be more useful than an .onion one (even though it's also likely to be blacklisted soon).