Because joining Rome in the past meant complete upheaval and uprooting of their current existence. Additionally, a formal initiative implicitly guarantees a widespread support system they would not have had earlier. Now they don’t have to necessarily leave their flock, as a portion of their flock will likely convert as well. Furthermore, the Vatican, by doing this, has ingeniously provided political “cover” for the process to start, i.e., after a few trickle over, many more who are quietly in agreement will follow suit. The Pope has, in effect, “de-scandalized” the process.
That makes sense.