for historical (Arm didn't used to have an enumerable bus to find devices, in the way the PC has PCI) and economic (vendors want to do as little work as possible, so they hack up one kernel version to run on their hardware then move on to the next hardware) mainline kernels haven't been able to boot on Arm devices. some absolutely heroic work has happened over the last fifteen years or so and it's now much better and we're now reaping those benefits - hardware can run normal mainline kernels and so expect to get "security updates" and "features" over time instead of being frozen on one version or carrying massive invasive annoying decaying patches against mainline.
