There are so many factors that go into this.

Every indie game has its fall sooner or later (unless they expand their dev team accordingly) simply because one/few developers can't keep adding content consistently to keep the game relevant. Eventually it gets to the point where people leave because they believe they explored everything there is to the game, which can be done fairly quickly in Toribash. Lack of updates plays a huge part in why this game has lost most of the players.

Another reason is simply unpopular genre. Majority of gamers, particularly kids, prefer fast phased games full of action, which is why Fortnite is popping off right now. Toribash is like an opposite of that. At the peek of Toribash popularity you would most likely spend 90% of the time waiting in a queue to play, which doesn't appeal to anyone, so those who weren't interested in community side of things simply couldn't bare the wait.

Finally, the learning curve is too steep, and with alts being able to roam in beginner servers the learning curve is even worse. I can see how most people who find this game end up quitting in first hour of gameplay due to lack of understanding of the physics. This sort of falls back into the "unpopular genre" category, the game simply doesn't appeal to large masses of gamers.

I could probably ramble on and on about why TB lost its playerbase, but i'll wrap it up right here, it simply doesn't appeal to "average gamer"