Micromasters were made by Hasbro to copy/compete with Galoob's Micro Machines (a bit like creating Kreo to copy/compete with Lego).
But just like Kreo, Micromasters didn't last, and around the 1990s, Hasbro started buying up a number of smaller toy companies instead of competing with them.... which included Galoob (Micro Machines), Tonka, Kenner (Gobots & Star Wars), and a few others.
So now brands like Micro Machines are owned by Hasbro, and it's just been a while since they last sold them (I think the Star Wars Titaniums in around 2008 were considered "Micro Machines" as they had Galoob on the packaging).
The reason for the Star Wars Micro Machines this year is because Mattel gained a license to do diecast Star Wars vehicles under their Hot Wheels Brand, and Micro Machines from Hasbro was probably allowed by Disney, as long as they are "micro" sized so that they don't clash with Mattel's license (Hasbro can produce action figures and vehicles with their exclusive license, but Mattel must have gained a license to exclusively produce diecast vehicles that are Hot Wheels sized).