It's part of Roundabout's mission to produce revivals of shows that could not otherwise be seen because commercial producers aren't interested in them. They can afford to do them, because they are a non-profit, and because they have a large subscriber base who buy into that mission. And, because of their limited runs, they can get big stars to do their shows who might not be interested or don't have the time to commit a year to do a commercial run (granted it's successful).
I doubt On The 20th Century, and even She Loves Me, would sell nearly as well without the A-List cast that they had, and they couldn't sell out the massive touring houses. The American Airlines and Studio 54 on the smaller end of Broadway houses, and a large percentage of the seats in taken up by subscribers.
Every once in a while they will have a big hit like Anything Goes or Cabaret, that gets extended several times (by more than just a few weeks) and shows that it can sell without the subscribers. (Actually, I'm not even sure why AG was at Roundabout, I feel that's a well known enough title to justify a commercial revival but anyway).
I love what Roundabout does, and I love a classic Broadway show, and I wish that more people could see them, but it's not what they are trying to do. I hope though that the She Loves Me stream was a big success (I don't think they ever released any numbers like DLL did?) and they might do more of that in the future!