As far as I can tell, on the books it’s never explicitly stated what the official reason for the assignment of Arakis to the Atreides is. There are some remarks on the CHOAM being the one that assigns it to the Atreides which, in turn, is stated to be “controlled on the 56.7% by the Emperor” (or close to that effect)
It is generally implied although never explicitly stated that the Leto is a reference among a lot of the Houses that expect benefits from the CHOAM which derives a big part of those benefits from the spice.
This is later on explicitly confirmed on the latest Dune 2021 movie on the fact that the Emperor was official taking the control away from the Harkonen due to the insufficient performance of the mining (hence reduced benefits to the CHOAM). This this assignment was a reward for Leto and a punishment for the Harkonen.
This seems to be rooted on the hints on the books and, while never explicitly stated, it makes sense so I tend to take it as the official cause. Obviously the REAL motive is to get rid of Leto as he was becoming too popular.