According to the table here, the Kingdom of Serbs, Croats, and Slovenes had almost as many Muslims/Bosniaks as Slovenes, and no where near as many Slovenes as Serbs or Croats, but Slovenes got included in the name, and Muslims/Bosniaks (and Macedonians/Bulgarians, which also had a significant population) did not. Why? Was this a fairly arbitrary terminology issue, or did it signal that ethnic groups not represented in the name of the country were considered outsiders, or had less political power, or something like that?
Same question for the State of Slovenes, Croats, and Serbs. I haven't found ethnic data for it, but I'd imagine the primary differences from the Kingdom would be way fewer Serbs (maybe comparable to population of Slovenes or Muslims/Bosniaks?), and almost no Macedonians/Bulgarians.
There is a multitude of reasons for this that have to do with politics, national identity and religion.
National identity: by the time the SHS formed Slovenes, Croats and Serbs had a strong national identity and movement going with it. In the case of Bosnians that wasn't the case at the time as Bosnians were considered people of the region of Bosnia. The term Muslims was used for the people who would now be considered Bosniak. Note that Croat and Serb nationalists often considered them to be just muslim Croats/Serbs.
It's a bit different with Macedonians as their national identity was already begining to form, but wasn't quite at the same level as the main three. More importantly however, is that Macedonia was conquered by the kingdom of Serbia and did not join the union voluntarily; Serbia "joined" SHS as a Serbian state and kingdom, not as a territory inhabited by South Slavs.
Apart from the two there were also several other major ethnicities that were marginalized such as Germans, Hungarians, Albanians and Italians.
At the time SHS was formed, the idea was to form a South Slavic nation on the wings of Illyrian movement of the 19th century with the three groups at its core, but by the time the kingdom of Yugoslavia was proclaimed, the state was becoming more and more just Greater Serbia.