1. They faced inferior armies technology wise; Air superiority

was obtained. No enemy fighters could threaten their fight-bombers, which provide supports to their matching land forces.

2. Those attacked countries are relatively small and thus have no defensive depth. No strategic bombers are needed. Attacking a huge country is different story.

3. I believe that those missles have range of hundreds of miles while large caliber naval guns can reach tens of miles. Those are powerful enough to eliminate coastal guns and bunkers (so the marines have more chance to survive when charging to the beach). They could also target defenders' artillery battery and reinforcement.