ACC is not a successor to AC for me, it's a different game. Both have their roles to play but without doubt AC holds more appeal to me for the wide range of cars and tracks it covers. We have both Kunos and the modding community to thank for that. 8yrs on and it's still an amazing game.
Because ACC's content is boring to be personally. I do not like to be limited to just current GT3 and GT4 cars. I never bothered to download modded cars for AC as I feel like most of them aren't even good but track mods are amazing and I have tons of fun with just the original Kunos cars on different tracks.
I also love the selection of original cars that Kunos gave us, we had vintage and modern GT cars and timeless classics like Mclaren F1 GTRs, Porsche GT1, Mazda 787b and then when you are feeling for something different, hop on the Lotus 98T which is a good representation of legendary cars in 80s F1 like the Mclaren MP4/4 which is rarely seen licensed in any good Sim.
Also, AC has a laser-scanned Nordscheleife, no sim is complete without the Nordschleife imo.
Many reasons really, the VR implementation is appalling in ACC, plus it's graphically inferior to AC, it lacks content, variety and mods - plus I'm a lot more interested in older/vintage cars these days. So, no Feldbergring, no party.
To be fair, I occasionally enjoy a bit of ACC sometimes. We shouldn't forget that Kunos has delivered what we have missed for some Years- A Sim that accurately simulates a whole series, with all it's Tracks and Cars, instead of the somehow scattered content of AC. On the other Hand, I simply don't have enough dedication to drive thousands of Laps around Monza in a GT3, just to squeeze those last 0.0001 seconds out.
I can, however, drive thousands of Laps around Fonteny in a Lotus 49.