As with many things in life, there's no one simple reason for this, but I can think of a few factors that probably play into this.

One is that 50% of people are worse than the average person at any given thing, and in this case, I think the 2 relevant things would be tech literacy and concern about things like privacy. As Gen Z has entered the workforce, businesses have apparently begun to express concerns over the lack of tech literacy among applicants, especially when it comes to stuff like desktop software. Which makes sense since Gen Z has grown up in a world where basically everything is app based and on your phone. I've worked with highschool-age kids who never had a desktop/laptop in their house growing up. Add to this the average person's concern on digital privacy and other concerns like the corporate greed that shut down the third party API access - which is basically nonexistent, since many people have trouble caring about something until it personally affects them - and it's easy to imagine that your average Reddit user, especially lurkers, probably uses Reddit through the official app on their phone and doesn't even know any other way they potentially could use Reddit. Most of them probably don't even know what an adblocker is, let alone an RSS feed or something. I believe Reddit themselves said that the people who used third-party apps made up like a fraction of a percentage of the user base, with the next smallest group being those who used the website, and that the majority of people used the official app.

Then you have the force of habit. Habits, once entrenched, are hard to break. People like consistency and are often opposed to changes in their routine, especially ones that don't have an obviously beneficial aspect to them and ones that will require some amount of effort to change. This is why doomscrolling social media in general is so difficult to stop. You can change platforms, but it takes a conscious effort to choose to stop entirely.

So your average Reddit lurker probably was upset about the change, but since it didn't personally affect them, they grumbled about it and then went on with their doomscrolling.

Then, you get to the users who actively interact with Reddit, either through posting content or commenting. These are the kinds of people who aren't going to just stop interacting with the subreddits they're in and become lurkers, and they have even more reason to be stuck in the habit of using Reddit, because they have the emotional weight/connection to the communities they frequent. These are the kinds of people I was thinking of mainly for my previous comment, because they're the most likely to stay despite being disgruntled with Reddit's recent actions. It's like they're using Discord even though they think it sucks compared to a similar program because all their friends are on Discord, and all their friends are on Discord because they're on Discord.

There are also the people who financially depend on social media. People like artists depend on platforms like Reddit because their livelihood depends on them putting their work in front of people's eyeballs, so they can't simply leave the big platforms. They might establish themselves on places like Lemmy, but they won't be able to abandon Reddit simply because that's the kind of place where the people are.

All of this to say, that basically the tipping point for the mass abandonment of Reddit will only happen when it personally negatively affects the majority of people, or when the better alternatives have a sufficient amount of users/content to make the effort of migrating seem worthwhile. Right now, Reddit just has too much gravity from being established for so long and having such a large userbase to keep your average person from wanting able to leave.