How about the fact they gravitated toward Aikido as experienced martial artists and didn't reach the level of O'Sensei. What attracted them to Aikido, as experienced martial artists, what was it in Aikido for them they didn't have?
I suppose some of the founder's students approached him looking to learn more, without attachement to their previous training and emptying their cups, and I suppose others had different things in mind. Some were sent to study with him, like Mochizuki, others were approached by O Sensei, like Abbe ( yes, O Sensei looked for new students, what a surprise), and surely there was people who didn't liked what they saw and moved away.
It's a case by case and every one of them deserve their own thread to avoid generalizations. The motivations of i.e. Tohei (who has Judo experience iirc, but also went to study under Nakamura Tempu) and the ones of i.e. Mochizuki (who was sent by Kano to study with Ueshiba) probably were a bit different.
You really have to see Aikido as a complete art. Maybe not as brutal or lethal as some other arts. But as equally humane as most other modern martial art of its time. I don't think we should measure an art by its brutality, but by its humanity. That is evolution.
Complete?, I don't think so. Valid, interesting and worth studying yes, but complete?... I doubt it.
Btw, arts are not humane, people is and evolution (and its mechanisms) is a different thing.