I am glad that you understood the deeper aspects, Lang!
We can look at this from many different angles. I will briefly discuss two.
1. The density of matter is highest in kama loka. That is where pathavi, apo, and tejo are strongest.
– As we go to higher realms (six Deva realms), those births arise with much less lobha, dosa, and moha; thus, the density of their bodies and environment is much less. That is why scientists can’t see those realms.
– Births in Brahma realms are possible at even smaller lobha, dosa, and moha levels.
2. When an average human attains the Sotapanna stage,lobha, dosa, and moha reduce substantially to raga, patigha, and lower levels of avijja. Thus, pathavi, apo, and tejo in the suddhatthaka generated by the Sotapanna is much less in proportion. Accordingly, any anger, greed, and ignorance arising in a Sotapanna are not enough to engage in highly immoral deeds. That is how a Sotapanna avoids rebirth in apayas.
– By the Anagami stage, that trend will lead to the disappearance of kama raga and patigha. Thus, an Anagami would not act with anger or kama raga. Thus, pathavi and tejo in the suddhatthaka generated by an Anagami would be much less proportioned. That is why an Anagami would be born only in a Brahma realm.
– By the Arahant stage, no gati is left to generate any kamma bija (or hadaya vatthu) with even a trace of suddhatthaka. No rebirths for an Arahant.
That is a very brief outline.