Probably an awkward question to answer, especially here, you may be better off asking officially. I would imagine size is the most likely push as with modules the phone is bigger anyway. I’ve only ever had the FP3 so can’t compare. There’s also environment and economy, smaller contacts, less gold is required.
I don’t see reliability playing a role, nor compatibility. From my understanding all the modules are in house design so they do not have to be compatible with any other product.