Simply put, they're all the same shit. And even the newer indie attempts still somehow manage to be the same fucking shit. No attempt has been made at innovating the genre save for Quake Champions, which only innovates at face value and doesn't innovate arena shooters as it is not one itself. The pros won't bother playing anything other than CPMA because all the newer entries are fundamentally still the same shit and fundamentally play out the same with only minor differences in execution in terms of weapon loadout and movement tricks, but the very core of high-octane movement, situational weapon arsenal and item control remains completely unchanged and unmessed with, so most people end up sticking with the most popular and tried and true example (Quake 3).
This happens because these newer arena shooters are made by old pros, and old pros who are incredibly versed in the technicalities of a game like Quake 3 know what makes it tick, but also wouldn't dare fucking with it, and thus don't really think outside of the box. Any differences their end product ends up having are mostly aesthetical or in terms of user experience, and from a very high level the new twists on the Quake formula are set in stone only which only high level players may discover and appreciate because these nuances are only noticeable from a high level. Why would most people here care if you can jump on one toe to increase your airstrafing speed when the rest of the gameplay turns out to be exactly the same as its peers?
The amount of active (potential) arena shooter players isn't that high to warrant financial success for multiplayer-only games in this day and age where people mostly play the most popular online games only because they're the most popular. From that standpoint it'd make more sense to have some kind of singleplayer campaign so your game has value outside of its most likely dead multiplayer base, a campaign in which you'd have to innovate in some way to stand out on the market since Quake 3 doesn't translate very well to a singleplayer experience, innovations which could then be backported to the multiplayer, and maybe create something new. Even fighting games manage to have more variety than arena shooters.