There are multiple reasons:
1. Yandere School, at the first glance, looks like shit. For example, in Jay's last Yandere School video I saw, the main character is tilting almost 45 degrees when turning while running and there doorways are made for freaking giants:
View attachment 277565
The rest of the assets (models, textures, animation, etc.) also looks bad. It's your standard Steam Greenlight shit. As for whether the gameplay is good, it doesn't matter when people dismiss your game based on the visual aspect alone. I surely did.
2. The name "Yandere School" is probably the most unoriginal thing that could have come out of the developers' heads and it doesn't stand out. I mean, the BijuuMike's Yandere School video series is called "Yandere School Simulator" and Youtube Gaming tags almost all Yandere School videos as Yandere Simulator.
3. Virtually nonexistent marketing. All they did is sending review copies to some youtubers. Meanwhile, Alex is putting out two videos a month even when he has literally nothing to say, droning constantly on pointless lore of the characters and gameplay features he will never implement, and making edgy jokes. Also, he got a bit more famous during the Twitch drama, getting even the more serious gaming youtubers and video game journalists to notice him.
4. Yandere Simulator has been out for much longer. The characters became recognizable and people got attached to them. The game is free and every amateur video game youtuber used that game to fill the channels. Even people who hate weeb shit have heard about it.
5. Yandere Simulator has tons of Youtube bait, including memes, debug features, "easter eggs" (not so hidden for easter eggs) and recently also literal youtuber pandering, so Yandere Simulator videos are easily digestible, and therefore fun for the attention span of an average Alex' fan.
4. Yandere Simulator was there first. Anything that comes second feels like a ripoff and does so until it's proven to be good on its own. (Of course, the game idea was original and fresh in 2014 because it wasn't Alex who came up with it, but some random /v/irgins on 4chan.)
5. Yandere Simulator is not only the game it is now. For many people, it is also the game that has been promised to them and is yet to come out. Why buy a shit game that is available now, when you can play another shit game for free and wait for it to become a glorious epic edgy weeaboo masterpiece (
)?