I have honestly found running other games like Shadowrun, Vampire, WEG Star Wars, and others more challenging than D&D in any edition. We had fun and have played them on and off for decades, but they never have engaged me as much as either player or DM.
I think that's a broader issue!
First, it's simply harder to find willing players - I want to start a 5e game -boom 6 players easy and I often have to exclude or split off a few. Try to start a Star Wars, deadlands etc. game? Hard to get 3-4 players interested.
Next (there was a thread on this a while back), much as we complain about the levels approach - it seems to REALLY engage players -they want to see their character at level 2, level 10, level 20 (even if actually getting there is unlikely). Systems that just give points to allocate to stats/abilities/perks etc. just don't seem to engage players as well (even if the resulting benefit to the character is similar).
And also, D&D has ALWAYS had more adventures available than just about any other system (I can't think of any others that have near as many low level, mid level or high level adventures), most just have an introductory adventure. Some have a few extra - but just not near as many. This makes it much easier for DMs to get into it and to get their players into it. Even if they eventually homebrew, there is just SO MUCH to borrow/steal from.