That is not unique among the Aiki arts. Takeda had thousands of students, but we mostly remember Ueshiba, Horikawa, and Sagawa.
I would call this a fault of Ueshiba for either not understanding how to create a system to pass his skills on, or not wanting to pass his skills on. This is essentially why I think chasing Ueshiba's skills directly is basically a cheeky endeavor.
But the other thing to remember is that a thing like Ueshiba's skills, of which we have mostly biased first-hand accounts, is apt to be inflated in power and import over time. Ueshiba is literally a legend.
The names you mention all had aiki which is nowhere to be seen in modern aikido.