They chose to join Italian Somaliland not Somalia, Somalia is the product of British Somaliland and Italian Somaliland entering into a union.
Regardless of one being more developed, if Somaliland wanted to be a separate state from Italian Somaliland they would have been able to.
Look at Djibouti(French Somaliland) very scarce in resources but independent.
French Somaliland was groomed to be independent from the beginning like Somalia Italiana.
Britain never prepared Somaliland for independence.
That's the difference.