The average user on the internet does not really care about the horrible changes or the ads served on the platform. That type of users make up the majority of the internet, so frankly it most likely won't be mainstream anytime soon. It might get big, it might become popular as an alternative, but as long as the internet is mostly made up of people that aren't much knowledgeable about certain things that people are in here, it won't.
Well, yes. But Reddit is "easier" to use than Lemmy. For example, Lemmy is in the fediverse, that alone makes Lemmy hard to use for absolute newbies that know nothing much. The average user would expect something similar to Reddit, when they are required to browse instances and find out that there are "multiple" lemmy websites and such.
On the other hand, they also have Reddit. They just visit the website, register, and it's all done. And then there's also the recommendation engine. Most of the people are lazy, they most likely would want to get things that they enjoy automatically as they browse the platform. Lemmy doesn't have such a thing, they might open the website, browse a bit and then maybe find out that there's nothing interesting being thrown at them automatically. Plus Lemmy is kind of buggy and the app is a bit "classic-looking". Also it doesn't have much features. So my guess is that before Lemmy can even gain major attention, some other corporate will see Reddit dying as a chance to make a copy of it in mere time, advertise it, and leave platforms like Lemmy in the shadows to rot.