There are numerous reasons. For starters, the routes were unknown and people didn't even think it was possible to circle the globe as various water routes had not yet been discovered. Another issue was technology. The ships were not as strong enough to survive such long journeys. Another was resources, as people did not have resources to travel so far. It was only during the age of exploration that kings and princes started giving money for such endeavors.
The primary difference between these two wars was that in the French and Indian War the colonists (who weren't that concerned with territory) were fighting alongside the British, while during the Revolution they were fighting against the British.
