I have been quite deep in AC modding before I felt like I need more of physics and features and moved to rF2.
This is why AC is superior in terms of modding:
1. It is more simple, less advanced.
2. It has easy compatibility to work with fbx files, so wide range of 3D modeling programs fits the task well.
3. Less encryption. It is easier and more inviting to use official sources as assets or as examples.
4. Better marketing and product completion. The game just had much better official content to begin with, more complete. The game itself also was more complete for what it is meant to be. The road has been walked through for modders just to easily follow, albeit it is much more narrower, shorter and straighter than rF2 which is superior simulation. Besides that they used some mods to official content, and that made a huge boost community wise. At the beginning modding was incredibly integrated into AC and officially suported. The official forums were insanely active and simply buzzing from activity, before it actually became too much for Kunos, and modding community was forced to leave to Racedepartment.
5. AC is much more popular. And everybody knows how it goes. Things that are popular attracts even more attention, simply because they already seem popular. More attention, more modders, more people who appreciates and uses their work. rF2 needed better UI to begin with. We got it now, but juices barely got squeezed again. There isn't even no indications for your best track/car combo laptime, so it is still simply just driving, you absolutely don't accumulate no records at all in rF2, there is 0 gamefication, no fun steam achievements....
6. The modding standard. Although I think in AC modding standard continuously went lower, at least in terms of RIPS and casual physics. IMO at least in AC there has always been a difference if mods are made up to certain spec. While in rF2 community there is little difference if shit is made, or someone makes an incredible work. Community seems to have robotic appreciation that doesn't reflect quality. On one hand nice - not that much needy complaining. On the other hand, no use of trying hard and trying to make something truly good that rF2 is acutaly capable of having, and this is while it is already 4x harder to make something decent quality in rF2, which reveals all rF2 power.
7. Uncertainty of rF2 development. People seems to be loving thing that are getting love. rF2 is getting love for a year or so, and then development halts for a year or two because devs are developing something else. Inertia never builds, and something is wildly WIP anytime.
8. LOL GUYS HAVE YOU TRIED TO DRIVE A CAR IN LATEST DEVMODE ?
AC modding actually grew out the sim itself. After AC development stopped, modders continued on developing. And everyone knows how crazy lengths it has went. Up to a point that people probably praise modders more than Stefano. Which to me is ridiculous. This proves that perhaps AC has been even too simple and too accessible if it could have been hacked like that, while in rF2 it is not even possible to break encryptions.
THE MAIN REASON IMO has to be ease of work with FBX files. While rF2 gJED project was axed I'll never understand. It just shows how low of the priority was modding for rF2 developers. Which is perhaps correct attitude having in mind that everything else have had to be completed further before modding could flow nicely.
The whole thing is so stupidly funny and wrong. AC had modding in mind to begin with having in mind that it won't be massive success, and there won't be that much official content. Thus big official support for modding at the beginning. The funny thing is that it is much more needed for rF2, but it never happened and it probably never will up to a spec that should have been. Yet rF2 has quite a lot of mods anyway. Quite a few very well made mod tracks. However way too few truly well made cars that would be scratch made and would shine in every department - looks, physics, sounds.... it just never truly picked up for rF2, and frankly IMO there doesn't even seem to be enough of community drive - which is perhaps most essential drive for all these things to happen. Making these creations for FREE, is already insane, IT IS INSANE. Creators deserve at least REAL respect and involvement. But there is neither.