The problem is that if you can't really just standardize every subsystem unless you're willing to give compromise your mission in the name of a one-sizw-fits-all approach. Presumably the people who built JWST passed on of-the-shelf busses for a reason. Imagine if you to tried to force JWST's bus onto every satellite:
people who don't care about pointing accuracy end up spending zillions of dollars so they can have fraction of an arc-second precision
satellites that are in Earth orbit have different thermal needs than one in L2
satellites that are less weight sensitive waste money on carbon fiber
an instrument that needs to be cooled down to 50K doesn't need a JWST-tier cryocooler, etc.
You could try having a range of things, e.g. small, medium, and large bus with a handful of interchangeable subsystems. But you're still asking designers to sacrifice their precious size, weight & power requirements
people who don't care about pointing accuracy end up spending zillions of dollars so they can have fraction of an arc-second precision
satellites that are in Earth orbit have different thermal needs than one in L2
satellites that are less weight sensitive waste money on carbon fiber
an instrument that needs to be cooled down to 50K doesn't need a JWST-tier cryocooler, etc.
You could try having a range of things, e.g. small, medium, and large bus with a handful of interchangeable subsystems. But you're still asking designers to sacrifice their precious size, weight & power requirements