I'm new to Tarkov but, just have some thoughts about why it takes ages for new Tarkov maps:
Developing mechanics and levels concurrently results in a lot of restarting, iterations, and corrections. Maps are meant to serve as a vehicle for core game mechanics. If you start designing a map in 2019 whilst other developers are coming up with new mechanics or changing existing ones, it'll be necessary to constantly be altering your designs to fit those, and since Tarkov is an early-access beta, this will happen a lot. They don't have the luxury of finalizing mechanics and designing levels around that.
Varied environment presentation means a lot of new art assets need to be made. Tarkov does a great job of giving maps in Norvinsk a different feel and look, hence the art team making whole new sets of props and architecture instead of recycling what is already there (Although they still do this, as well as taking advantage of Quixel assets)
The high-stakes nature of the raid/BR of the game makes bug-testing paramount. Dying and extracting should be the only ways to fail or succeed in a raid. The most frustrating thing in the world would be to be full of loot then get stuck in a pile of props with bad collision and run out of time in raid. Tarkov maps are large and extremely detailed- testing, fixing things, correcting holes in geometry, and any variation of error checking probably takes as much time as building the map itself. I worked on an FPS title that had similarly sized maps but had your standard kill/respawn mechanic and testers would still come up with 80 tickets in maybe two days showing where a player could get stuck or get someplace they shouldn't that a designer or set-dresser wouldn't be able to see. I feel like Tarkov has a very low tolerance for things like that considering the style of game; even a small snag on the ground could ruin a player's experience in game. On top of that they know that of the thousands of players who hop into a new map for the first time, probably 1/5 are immediately going to search for exploits.
