imo
gamemodes are bad, this was a huge issue for diabotical and you could see it in the gamemodes new people played/liked. Most of the positive feedback came from the 1v1/2v2 gamemode and wipeout, or at least that's what you would see when you opened any random players stream. Serious 1v1 gamemodes are not fun in afps, they work in fighting games because you will sometimes be sitting next to your friend playing with them, not true in quake. All of the traditional team games modes require timing, which is not very fun to learn especially 25/35s timers can be a pain, and the 30/30 that exists in qc is often times fairly painful in certain cases because it means you can run the items infinitely without a player ever having to delay an item.
When I started playing qc I was OK at timing items just by vibes from cpma, which is a thing players will learn, but it's not a fun journey to learn, imagine playing 6s but you always have uber disad and you don't know how to stop that from happening. Luckily, timing in sac is largely irrelevant, and the game mode itself is so foreign to the ones commonly played previously tdm/ctf/ca that it was a pretty decent gateway drug. A surprising amount of na players in qc (not that it's a large community) can trace their origin back to sac.
Wipe-out in diabotical was a good experience and I think it hooked a lot of new players for awhile until they realized how much of a mountain there is to climb to even play the game in the other two game modes they had in rotation. There was some other weird stuff going on with menus and matchmaking that diabotical had some issues with too. However, it felt to me diabotical was this game for practicing aim for most people because there just wasn't this long term goal to invest into or this addicting thing to play, for most people simply practicing aim will get boring at some point.
basically what nintails says here, but I would extend the logic to casual players as well
NintailsQuake doesn't appeal to a casual or competitive audience anymore.
If you are a competitive player the question you need to answer is: "Why would I invest my time into quake, when I could invest my time into any other competitive FPS game and get x100". Like why would any 15-16 yr old spend time grinding quake - when they could grind CS, Valorant or Apex Legends?
there's also not enough focus on casual experience, for most first time players loading into a server with 8 players or less is going to be a test unless you get lucky and everyone else is new to the game. TF2 solves this issue by having the default pub game mode having 24. Even in ql as it is now, the pubs that are played are a 24 Player FFA server and a 10 v 10 CA server. There are a couple of other thing going on every night but those are usually organized and played by players with a higher ELO (CA on those servers for instance has an ELO gate because having a low level player in them would be awful for everyone). edit: also being defenseless on spawn is one of the dumbest things quake players hold onto for some unknown reason, one of the best things qc ever did was give you the option to spawn with a shotgun or nailgun, and all of the starting weapons are a threat along with your stack.
Sidebar: QC has a terrible ass engine, and playing above 60 ping in QL is awful because the engine is also kinda crap (I don't think it's possible to run the game over 250 fps, and the game starts getting real buggy with physics and sound when you do play it at 250). From what I can tell DBT is fairly dead, despite it having a pretty solid netcode, but it might just be because I haven't interacted with it. The game itself just seemed to being trying to be Q3 but again which I don't really think is as appealing to large player bases as Q3 players think
it's largely just game modes bad, DBT seemed to really treat the design of gamemodes as something disparate from the basic core of gameplay that it needed to nail rock solid and like really fell flat cause of it, and I think DBT had decent momentum at one point to hold a steady player base if it had really been the experience people wanted. QC you can straight up just blame on the engine being awful imo, playing that game a lot of the time feels like you are playing a sim that is playing a game. There hasn't really been anything else that has scratched those two in terms of potential for a player base.