Honestly I don't know why there are so many almost identical IDEs.
I think the differences are more akin to the old Visual C# Express and Visual Basic Express IDEs vs Visual Studio. Visual Studio was always "everything", but you used to get the express versions that were "low cost" or free. They only had the single language in them. They were customised to just that language. This is what Rider is to IntelliJ Professional, except from what I understand, the plug-ins for IntelliJ are not always on a 1:1 feature parity. This can even be seen with Android Studio and IntelliJ Community. The Android tooling in IntelliJ Community is almost the same, but it does miss out some stuff in Android Studio. Because Android Studio is specifically for Android development, and IntelliJ Community is more general purpose Kotlin and Java development. I think the Kotlin Native support is slightly better in IntelliJ Community.
I feel that Rider is somehow better than CLion at c++, even after CLion Nova (Intellisense based on Resharper backend) became a thing.
One difference is that I write boost::asio in CLion, and just vanilla C++ in Rider, and before Nova it was completely unusable with async code, now it's usable with async code, but after a few days of running the editor I end up with fatal IDE errors for CLion, and never for Rider.
Probably a reflection of internal organization to avoid product teams stepping on each others' toes.