Every other colony of Spain was either given nominal independence or taken over by the United States. Why was Spain permitted to retain this one? Was there any discussion about it among American foreign policy makers? Was it brought up in Treaty of Paris negotiations?
Consider the areas that the US gained. Guam, Puerto Rico, and the Philippines, with Cuba becoming nominally independent.
All of them were important for use as bases for the USN. They provided coaling and harbor facilities in areas that the US had a strategic or economic interest in, namely the Caribbean and Western/Central Pacific.
No such interest existed for North Africa, the US had a minimal naval presence and were already becoming good allies with Britain.