Not sure there is enough data out there to have a clear picture of things, but Warzone does have a high cost of entry, and you can have a look at L4V.R0V strat community data to see how few make it:
https://docs.google.com/document/d/1InZl_jzlBz9_hMG4Vl_axx65p480opfyqTg9QwLqY-g/edit?usp=drivesdkso having a more user-friendly path to where the game is enjoyable is probably part of the answer.
Not sure it will ever reach the number of players some more casual games can, not sure it's a fair comparison either, maybe something more fair would be to compare it to a subset of board game arena's games.
Idle is probably something that could be compared to those, with lower cost of entry, but even then, those game are not the work of mostly one man, have large companies behind them with marketing budgets and don't have another game from a well known franchise with the same name that makes you go to page 5 in Google to find it.
With respect to the retention rates I'm not even sure it is that bad, it's the only gaming app I haven't deleted from my phone and I don't know how many I've downloaded, played once then uninstalled.