It couldn't be returned to a country that didn't exists before that places belonged to Spain. Its not the same thing, people that lived in Gibraltar was moved out. UN agrees with that, Spain politicians are to dumb to lobby anything. Check how much Spain spends in lobbying in US and how much Morocco (that its not even a democracy) spends.

“But Morocco didn’t exist!” - addressed in another reply, see sibling.

The UN argument boils down to Spain lobbying the UN early on (post-WW2) at a time when Morocco and other North African and West Asian states either did not exist yet or did not have the ability to represent themselves well at the UN.