I.e they started out thinking "Well we focused on Daedra last time, so don't spend too long on them, in fact given the plot of the last game they'd probably be summoned less."
Then they got to conjuration/summoners: "Right, atronachs are the classic summon, get them in so conjurers have a weapon, and we can probably balance them easily"
It won't have been til they decided to put the two Dremora outside Dagon's Shrine that they decided to make them summonable; basically they weren't gonna put creatures as summons unless they roamed elsewhere (Atronachs, being the classic summon were the exception) and having them roam was weird after the lore, so making them is more effort than it's worth, so don't add in the summons and we don't need to waste time modelling them.
It's a shame but that's how games are designed often :/