By definition they were never “Jordanians” which is sort for “Transjordanian” (on the other side the Jordan). While many Jordanians are Palestinians, the reverse isn’t true. Almost noone in the West Bank or Gaza had roots in TransJordan.
100 years ago they were referred to as “Leventine” and the group included those whom we now call Lebanese and Syrians – all of which were one group. However the British and French conquered the area, and created boundaries and for the last century they have all had different histories, which is why a Lebanese Arab and a Syrian Arab and a (Trans)Jordanian Arab and a Palestinian Arab are now different groups – even if they all sound the same.
One asks why people in New York are called New Yorkers when in fact most of them are something else (Europeans, Asians, Puerto Ricans). Believe it or not, even period of less than century creates new groups and identities.