What legitimate reasons are there to use Fuchsia instead of Linux? The only reason I can think of at all is to be allowed to make proprietary modifications to it.
Security
"Zircon is a capability-based, object-oriented kernel"
"The Zircon system fully isolates processes by default, and must explicitly grant capabilities and resources. Fuchsia passes capabilities and resources by handles rather than name, which leads to a system that only grants software access to what it needs."
