I switched from CodeIgniter to FuelPHP and its been super awesome ! just wondering why its not so known yet! i've been recommending it to many php devs.
Because a lot of developers follow the latest trends, and Fuel is an older and stable framework, and we're not very eager to constantly add new bells and whistles? Just guessing. ;)
The biggest issue with the framework is the fact that is uses quite a lot of static methods, which makes writing tests harder as you can't test everything in isolation. We hope to address that in the next version.