IIRC (again) they simply capitalized on a string of restaurants that lost Rubin for technical reasons (but not Kashrus reasons). At least that was the case for the three stores by the Mir that lost Rubin and got Kehillos. At the time Rav Rubin was downsizing, then something changed and he started getting new restaurants again.
I understand that, but i've seen it in many places. Many new restaurants also went for Kehillos instead of Rubin...
And also, why did those places that lost Rubin not go for Landau, Sheiris, or Bedat"z? The answer is pretty obvious, the lower price of chicken/meat and the lower standards of required Kashrus.