There are a number of possible explanations for Quest 3's higher retention, and the answer may even be a combination of multiple.

It could be due to the higher quality of experience delivered by the pancake lenses and more powerful chipset. […]

It could also be due to the improved passthrough. […]

There could also be a degree of selection here. People who are more engaged with VR are more likely to upgrade to Quest 3, and people who are willing to spend $500 or $650 on a headset may be more likely to continue to use it than those who paid much less.