Personally, Im more of a creative-mode player at the moment so the majority of the problems don't affect me. Its also been a couple months since I last tried Minetest (mostly on Mineclone). From what I remember, my personal dealbreakers were:

• IMO much uglier graphics, and more significantly, its harder to graphically customize with mods and resource packs. Maybe its more versitile than I'm giving it credit for, but at the very least, its not used as there aren't enough modders and artists making content for the game to even remotely compete.

• Performance - Minecraft may not be optimized, but I was getting much worse performance on Minetest. In particular, I was getting massive frame drops any time I placed or broke a block, making it extremely nauseating to play.

• UX - There were a ton of small roadblocks to actually playing the game as I wanted to. First, customizing the graphics settings - the menu was disorganized and defaults were really weird for my hardware. Then, I had to find and edit the permissions file to be able to fly and sprint on my creative world. Even after that, if I remember right, the controls or flight movement were limitted or weird but in a way that couldn't easily be fixed.

• Lack of world editting commands - Im sure theres mods for this one, but its more work to find, figure out, and set up and by the time I got to this point, having to put in even more work just to make the game comparable was a dealbreaker.