I can't answer this, but u/commiespaceinvader and u/kaisermatias have discussed this in these threads:
https://www.reddit.com/r/AskHistorians/comments/55nzcp/stalin_trotsky_freud_and_hitler_all_lived_in/
The consensus from these seems to be that Vienna was a large, cosmopolitan, multiethnic, and relatively free city in a central geographic location. Hopefully someone else will expand on this here as well.
Can you put up a map? I've found that "blocks" can refer to distances of several miles, at least when news articles talk about events in DC being just "blocks" from the White House or Capitol, when they may be miles away. Having a map may help determine if the answer is that there was something extraordinary about their neighborhood, city, or country.