Mobile suits in UC use part of the reactor energy to power and operate their thrusters. During the OYW there are certain higher end Zaku models with stronger reactors whose output numbers are about as high as other suits that can use beam weapons, but their higher thrust and mobility end up taking too much away from the reactor for it to be able to power beam weapons. In the case of the Zaku R type high mobility models, they sought to improve the performance by sticking on extra thrusters here and there, and sometimes building them with better reactors.
My personal headcanon thinks that upgrading a normal Zaku is a bit too inefficient to expect both high mobility and beam weapons too easily, so Zeon proceeds with Act Zaku and Gelgoog development projects which are essentially brand new suits designed from the beginning to be able to use beam weapons and have same/better mobility without needing addon thrusters.>I figured with at least the United Maintenance Plan suits...
United Maintenance Plan was more about spare parts and smart shortcuts wherever possible, and less about building mobile suits with the same abilities. Even if the Zaku II Kai and Rick Dom II share parts with the Gelgoog Jager, those two suits still can't use beam weapons. For one, the Zaku II Kai still has the same weak reactor as older Zakus, and it has even more thrusters (but much shorter operational time) so there's no way it can be expected to drive the power requirements of a beam cannon.
The Rick Dom and its UMP version Rick Dom II were better platforms though. They did have a fairly strong reactor to begin with, and some units were able to use beam bazookas but only after modification (and the fact that beam bazooka had a small reactor built in).