I would guess that this turn (which was rejected by Normis a few months ago but is certainly welcome!) was dictated by cost factors, the vast availability of code for ARM. This means that if they did try to support ARM-based systems that other companies build, they would have to pick and choose which specific boards to support, and they would be spending huge quantities of time and resources to ensure that RouterOS runs on somebody else's board flawlessly even though they have no assurance that they will sell a lot of copies to people who use those boards.
As cool as it would be if they did so, as far as I can tell, MikroTik has no real incentive to try to make RouterOS for ARM run on "generic ARM hardware" since, really, there is no such thing.
MIKROTIK RB3011 SOFTWARE
MikroTik's modus operandi when it comes to different CPU architectures has always been that they license software for generic x86 hardware (probably, at this point, more for historical reasons than anything else I would be sad but not shocked if, at some point in the future, MikroTik discontinues the practice of licensing RouterOS separately from a hardware sale), but for all other architectures, it only runs on hardware that they make that uses that instruction set.
Will be packages for other boards based on ARM cpu? (like x86 package).