ARA 2 is simply far easier to implement. The reason why so few DAWs supported ARA 1 was because it would have required major, important portions of most DAWs to be re-engineered.
It almost feels like it was specifically designed to ensure that it would be a exclusive to a certain DAW for a long time, and any other developer attempting to implement it would have to waste precious months of programming, testing, and bug fixing working on this one thing. Only Cakewalk fell for that and the (then already slow) development of Sonar slowed down greatly for about 2~ years as a result.