Fuchsia's development has taken a huge amount of time and the problem with trying to switch ChromeOS to Fuchsia is that if there are ANY regressions as a result (including to android support or crostini), a ton of people will be very unhappy. If there are any downsides, it would have to be WAY better in terms of user experience to make up for that, and I'm not sure that seems likely right now.
It also wouldn't solve the fundamental problem of ChromeOS which is that it doesn't have it's own apps (there were Chrome apps but they were deprecated) and only offers a somewhat imperfect experience for linux and android apps.
To overcome that, Google would have to already have worked out a new approach for native Fuchsia apps to commit to it (which would probably mean committing to Fuchsia replacing android) and I'm not convinced Google is capable of making that type of decision in its current state.
It also wouldn't solve the fundamental problem of ChromeOS which is that it doesn't have it's own apps (there were Chrome apps but they were deprecated) and only offers a somewhat imperfect experience for linux and android apps.
To overcome that, Google would have to already have worked out a new approach for native Fuchsia apps to commit to it (which would probably mean committing to Fuchsia replacing android) and I'm not convinced Google is capable of making that type of decision in its current state.