I'll admit this is a pet peeve of mine, but I'll acknowledge that I think many theme league creators (even some very experienced ones) don't really know how this particular setting works.
The issue is "minors salary cap", and it's one of the options you have to select under "League Settings" when you are creating a new theme league.
Some relevant background: This setting was introduced about 8-10 years ago by WIS as one of several tools to limit what used to be known as the "fatigue strategy" especially in OLs. They also introduced min PA and IP requirements and a few other tweaks. I don't think these changes were necessary or well thought out. And candidly, with the elimination of AAA players in OLs, the minors salary cap is no longer needed at all in my opinion.
How it works: when minors salary cap is set to ON, the owners are limited in which players, and how many, they can have in AAA at any time. Specifically, the total salary of players in AAA can never exceed 10% of your total team salary. Effectively this restricts your roster flexibility. Some players can never be sent to AAA under any circumstances if their individual salary is too high. Or you will find yourself unable to rest, say, 2 fatigued players in your starting lineup by sending them down to AAA, because combined their total salary may exceed the 10% cap.
Minors salary cap has NOTHING to do with the quality of SIM-generated AAA players. That is controlled by different settings. Minors salary cap ONLY serves as a mechanism to restrict owners' ability to send players down to AAA.
Important point: the default setting for new theme leagues is that minors salary cap is ON. This is a huge blunder in my opinion on WIS's part, made worse by the fact that many theme league creators either don't know what it does, or forget to turn it off.
In general, this setting should ALWAYS be set to OFF for theme leagues, unless there is a very very good reason not to do so. I'm not even sure what such a good reason would be, but unless you have one, you should be setting it to OFF.