They can't. I mean they can on their phones. But that's a tiny percentage of the total Android HW. All the big SoC vendors would have to get on board and rewrite their milions of lines of their kernel code on top of Fuchsia. Android is also not just phones, but also on tablets, tv boxes, etc.
This would just cause even more fragmentation in the Android ecosystem.
> All the big SoC vendors would have to get on board and rewrite their milions of lines of their kernel code on top of Fuchsia.
Apple's PPC to x86 transition was successful, as was their Carbon to Cocoa transition. If you want device driver specific examples, Microsoft's Win9x to XP transition was successful, if lengthy, as was its transition to WDDM for video drivers.
SoC and other hardware vendors go where the money is; they may grumble and drag their feet but they will make the transition if it's required of them.
This would just cause even more fragmentation in the Android ecosystem.