Those may very well have been the filmmakers’ reasons for not including a ghost of Ben.
However, showing his ghost is the natural capstone on his redemption story. Without it, it leaves the audience to say, “Oh, well. I guess he’s in Jedi hell.”
Also, I feel like the script already seems to address this (as though they originally meant to have his ghost at the end) by having Palpatine refer to Ben as “the last Skywalker,” as KumoNin points out.