I imagine multiplayer has slowed down for a bunch of reasons.
My memory might be a little hazy here...I left the online element of IL-2 some years ago...
At the time I left, mods had been introduced, but no unified mod packs existed, a different website was the center of IL-2 mods in the English speaking parts of the internet. Access to IL-2 modding tools was VERY tightly restricted, and you essentially had to be one of the authorized few to get them. Without unified modpacks, random "pick up games" between groups of people were limited exclusively to stock slots anyway. If you wanted to play a game with mods you had to organize beforehand for everyone to get exactly the same stuff or there was trouble.
It was the written policy of the other site never to touch or alter at all stock slot planes. Despite that there was a loud "mods are cheats, modders are cheaters, to mod is to hack" voice out in the community, I seem to recall the dev team trying to interfere with mods in several ways, can't recall for certain.
Again my memory might be hazy, but I seem to recall at that time that IL-2 felt "complete" only when playing Eastern Front missions and campaigns. I remember people complaining that the entire western front including Italy, Africa, etc, was limited to dogfight maps. I remember complaints that the entire "Pacific Fighters" thing felt terribly rushed and incomplete and people felt let down by it. There were no battleships or cruisers modeled other than the British ones which were used as "generic" battleships for the US, IJN, etc.
My own personal experience was that mods were slowly improving the game, but breaking the online community down into smaller segments. I had a few friends who had gotten into flight sims back in the days of Microsoft's Combat Flight Simulator. They had been waiting for *YEARS* for a West Front expansion and when things like the Cannon UK channel front beta map came out, or the "Italy Africa Greece" map came out, and we started modding our games with Spit MK1's and 109E1/E3s and stuff, we kinda broke off and did our own little online games.
Then the guys I knew and myself all got busy with graduating, going off to work, etc, and that was it. Just too busy to come back.
Edit To Add:
I should mention that other than mods there have obviously been other developments.
For some reason, I seem to recall that near the end of my time playing IL-2 online, I seem to recall that 'Cliffs of Dover' was released, a segment of the community, for whatever reason, left to play the new COD. I recall that COD was found to be quite horrible, and some of them came back to playing IL-2. But I have to imagine that every time a new sim came out, like Battle of Stalingrad, etc, people were pulled out of the IL-2 1946 community. Then when you had to consider that as patches came out, some of us stayed in our own modded versions rather than upgrade and start again, there were version switchers since different people played different patch versions, etc...
My feeling was that it just kinda fell apart into smaller groups and then, since there was no longer one big community but many individual smaller ones, it was easier for those smaller ones to die out one at a time.