RF2 fanboys will tell you stuff along the lines of "oHmAgEd, rF2 so complex, they can't understand it. They can't get close to it. The physics calculations blow our minds, it's the String theory, noone can approach this magic, they can't get their minds over it." Bla bla, that's typical. That stuff been cycling in this forum for a decade now.
Short and correct answer is: Modding in AC is more popular, simply because AC is much more popular. Not because "Modders can't handle the String theory ultra mega complex physics of Realismfactor2 bro". There's nothing modders can't do, mostly it's a work of passion.
RF2 fanboys (who have been waiting for the magic moment of RF2 for a decade now) won't tell you that RF2 was born dead, kept a very ugly and user unfriendly UI for a decade, looked bad and also ran bad (You typically pick either), even official content was very inconsistent, and then came along AC feeling amazing, looking amazing, running amazing. Base content being quite satisfying, consistent and high quality, all tracks being laser scanned, intuitive UI and people moved over, and so did the modders. They won't tell you that these were the actual reasons. They'll cope with "cUz mOrE cOmPlEx BrEh!" cuz it makes them feel better.
BTW at that time RF2 fanboys were claiming that RF2 tracks (none were laser scanned) were more realistic than AC's laser scanned tracks
Now, debunking the "mOrE cOmplex theReFore mOrE reAlisTic bRo" myth. More "complexity" or more "calculations" doesn't necessarily result in a more realistic sim. This is evident from the fact that cars in AC drive and handle much more realistically than RF2, also than ACC. ACC is supposedly more complex on paper, but when it comes to practice, it isn't more realistic. In fact it is less realistic than AC.
You can go ahead and dive into AC physics values yourself. It's not necessarily any easier, it's a different approach that is more efficient and gets you results.
Most people with irl experience will tell you that AC handles much more closer to reality. This is also confirmed by the long time simracer and F1 driver. His english malfunctions here, but what he means to say is "One of the best if not the best".
Here is yung Max from a decade ago using commercial sims with a G27 no less, to reiterate the fact that he doesn't just repeat what he heard elsewhere, the guy is actually experienced simracer and is able to form his own opinions by testing and feeling, using the best equipment available, also having used the older equipment.
And mind you, this guy has access to the F1 team sim, and still says that AC is one of the best if not the best to practice F1 or GT mods.
So to repeat the short answer, modding is more popular in AC, because AC is much more popular. You have 2% of the amount of the player base of AC, you get 2% of modders.