From a development standpoint, the daily expanding window is actually the simpler (and cheaper) implementation since they've already implemented it before with FP+, it allows both on and off-property booking window calculations to share code, and keeping track of all the valid date ranges for the varying length of date-based tickets that would be necessary to do it the other way would be messy and a maintenance headache.
I agree that it's more of a hassle for those guests affected, but it's likely it's just not worth it to Disney to accommodate, and even if the expanding window weren't the simpler solution, I think Disney would still rather slightly inconvenience a subset of off-site guests than upset on-property guests.