MSFS2020 is straight up infinitely more friendly to novice development. Instead of having to read enterprise documentation for a custom product that is entirely based on binary file types that is very very mid 2000s microsoft, you literally import a standard 3D model file into the engine itself, and manage it with built in dev tools that are only getting better and more powerful, and the vast majority of plumbing to make things work is done in human readable xml and cfg text files, though there was some of that in FSX

Adding visual things to FS2020 is infinitely easier than any version before it. Meanwhile for ground texturing, turns out replacing that with google satelite data is as easy as proxying the web calls to google servers instead of bing. Theoretically, you could add in your own tileserver that also responds to those calls if you have your own ground textures, though I think you can also manage those in the dev tools in the sim.

Meanwhile, the api for moving data and programming into and out of the sim is super similar to how it was done in FSX, meaning it's almost backwards compatible, and supplemental apps that interact with your flight data were out within months.

Another fun thing about FS2020 over the previous version is that the 3D cockpit is much easier to program, because the instruments and even TV screen style flight displays can be programmed in javascript and WASM, giving you multiple different ways to build powerful and impressive cockpits. For FSX, things that wanted smart and powerful cockpit displays, like a modern boeing, basically required pulling data out of the sim, rendering it yourself, and smuggling that data back into the sim, meaning updating the displays at 15 fps put a large overhead on an already struggling single process executable.

MSFS2020 is so popular, and so easy to access, and so inspiring, that it has infinitely more amateurs trying to build planes in it than FSX had over it's entire life.