I feel the two big reasons are:
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The average user of a site like Reddit probably hasn't noticed any significant changes; or if they have, they just don't see them as a problem. So they don't have any significant incentive to emigrate to another site. On the other hand, people who are tech-savvy notice the changes; and decide they need to move.
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To a lot of people, the Fediverse is just not as convenient as centralized sites. People who are more tech-savvy and/or use Linux, are willing to put up with a bit of inconvenience in exchange for using a site they see as better.
It's also worth keeping in mind that right now, the Fediverse is still in its early days. Every site in its early days generally has a broadly similar userbase- people who are familiar with technology and willing to put up with some inconvenience because they see the potential.