ARM is just an instruction set architecture specification. It is not any specific CPU; there are many different implementations (different chip designs). They can differ to various degrees, and often have at least a few non-standard features.
Hence, each ARM chip/platform often needs at least a little bit of special support (or drivers) to fully work in Linux, besides the standard ARM stuff.
Apple's platform is actually very different, with much more non-standard stuff, compared to other ARM implementations. Hence, much more extensive work was required to support them, than would be typical for other ARM platforms.
Even some of the really low-level features are different. For example, Apple have their own Interrupt Controller. Supporting it (in place of ARM's standard General Interrupt Controller) was a prerequisite to even be able to boot the linux kernel. Another weird thing about this platform is the bus configuration.
Also, Power management is something that is virtually always chip-specific, and the Apple M1 has some unusual quirks on top of that.
Hence, each ARM chip/platform often needs at least a little bit of special support (or drivers) to fully work in Linux, besides the standard ARM stuff.
Apple's platform is actually very different, with much more non-standard stuff, compared to other ARM implementations. Hence, much more extensive work was required to support them, than would be typical for other ARM platforms.
Even some of the really low-level features are different. For example, Apple have their own Interrupt Controller. Supporting it (in place of ARM's standard General Interrupt Controller) was a prerequisite to even be able to boot the linux kernel. Another weird thing about this platform is the bus configuration.
Also, Power management is something that is virtually always chip-specific, and the Apple M1 has some unusual quirks on top of that.