I believe the long-term goal is for it to replace Linux as the kernel in Android.
The technical reason, I'd say is that the access control model in Android is a kludge on top of the Linux kernel, and it is more difficult to sandbox apps. Fuchsia was made to support Android's model, and in a capability-oriented OS you get sandboxing by default.
A political reason is to not be dependent on Linux, but I'm sure that other people have opinions about there being more.
The technical reason, I'd say is that the access control model in Android is a kludge on top of the Linux kernel, and it is more difficult to sandbox apps. Fuchsia was made to support Android's model, and in a capability-oriented OS you get sandboxing by default.
A political reason is to not be dependent on Linux, but I'm sure that other people have opinions about there being more.