If we're talking specifically about passenger comfort, then in the specific case of Eglinton, there's also a pretty significant push factor for a minority of riders.
To board the Eglinton Bus, you stand on a kerb, then step aboard the bus. To disembark, you stand at the door, then step to the kerb. In a lot of ways this is even easier than doing the same maneuver from a car, especially if you are travelling with stroller-aged children or you use a mobility aid.
To board the Eglinton Crosstown at the underground stations, you've got to descend 3-4 flights of stairs and escalators, or take at least 2 elevators. It's a thing.
This thing will not be offputting to most passengers, and it has other compensations. (A dry, well-lit waiting area. Little coffee shops at certain stations. Bike parking. Off-street bus transfers. An LRT...)
But there will be other people (especially those who will struggle most with the flights of stairs) who might reasonably prefer a bus. For these people, the above-ground platforms meet this specific need much better. (And, indeed, they have nothing to complain about on the Finch Line or the ION.)