To be honest, all the visual quality is most probably done by the fantastic MSFS engine.
The same high-fidelity add-ons for X-Plane also has 4k textures and 1 GB in total size. The modelling quality is to be honest, is not too far off in terms of details. Welp, most XP airliners are even more detailed with their own explorable virtual cabins, openable cockpit windows & doors and such.
Model behaviour / flight dynamics is probably the second thing, as each simulator treat aerodynamics differently. Old MSFS uses data tables, which can be easily compared with technical data, but the 2020 one uses a “modern” X-Plane-like blade element simulation. I think most ambitious developers would try their best to take advantage of this new model, which could prove to be challenging.
Another plausible thing as has been shared by PMDG last year, is the very limited SDK, which probably didn’t allow as much customisation as a lot of developers would like to have. In the past, developers can just develop their own separate modules, simulate their systems outside the MSFS and interfaced using FSUIPC. They don’t need to wait for MSFS / Aces Studio to open / decrypt the MSFS simulation module for them to tweak over, because there were hardly any simulation modules in the previous generations of MSFS default engine.