I think the main reason is that the form/identity has to be something the shifter is comfortable with.
The first demon form is forced on the demon, between his/her imagination and the summoner’s imagination/picture/idea
Later ones, at least if they are going to be stable are ones that the shifter can concentrate on, and know the details of very well. And keep the form solid/unchanging.
I suspect if one were a taxidermist or veterinarian you could be a pretty good shapeshifter.
Most secondary forms are humanoid because they are closest to what the shifter knows best. I.e. a human with hooves, or claws or tentacles is a relatively minor thing, or just a completely wacky unreal monster that the user completely makes up.
Getting/knowing all the details of a wolf, and getting it right and realistic is much more tough.