The Italian powers/city states were at a disadvantage in comparison to the colonizing powers of the era in a few ways.

First is obvious which is geography. Being in the middle of the Mediterranean, they did not have the ports directly on the Atlantic that Spain, Portugal, Britain, and France all had.

Also, being in the Mediterranean, a lot of trade was done with the eastern Mediterranean and so there didn't appear to be nearly as major a need for expansion as they were already successful traders in that area. The other Western European nations did not have that proximity and saw expansion as a way to compete.

Finally, around this time, the modern concepts of nationhood were being formed in Spain, Britain and France (and of course Portugal and the Dutch etc.) and with it came a lot of economic support as well as military support for the colonization/conquest of the Americas. The Italian powers, while rich, weren't united (and Italy wouldn't be for many more centuries)