It was because the change required magic. Quite a lot, in fact. Most of a changeling's inherent magic was tied up in maintaining that form, and it left little to match the feats of other races. Pharynx had gotten the closest, but he'd still fallen short of a rainboom. And since he was as much a changeling's changeling as Dash was a pegasus' pegasus, she was certain noling else could do it either. 