That explanation raises more questions than it answers. What about the other Mediterranean nations? Why did any nation colonize, since the colonial powers usually had at least one rival European nation they "had to deal with"?
Well, I to does raise question -
but not those questions. For one, the traditional (major) colonizers were doing out of mutual competition with each other in a real where expansion, gold, and glory wouldn't necessarily entail fighting each other - and gains could be had for staggeringly less investment of resources than attempting conquests in the mainland. Which is exactly what most of these nations were really using their new world exploits to do - get a big, quick return on investment.
The traditional colonizers colonized in the way they did because it really the only option open to them - when Spain and France attempted conquests in Italy, it lead to a huge expensive debacles that ended up being more or less pointless beyond the prestige gains of controlling European soil due to the Ottomans ultimately controlling trade from the East anyway.
The second point would be
What other Mediterranean nations? the majority of the Christian Med Sea coast was locked up either directly by Spain and France, and the remainder by Venice or Austria. Sure you had an exception in Tuscany, but Tuscany was busy directing its efforts into being the cultural capital of Europe, and attempting to maintain itself as independent (or as Independent as possible) -and the Papacy seemed happy with conquest by proxy by any Catholic power. The remainder was Islamic - and over time largely falling under the dominion of the Ottomans whom probably could have set off voyages if they wanted to, but they had every reason to keep looking east for the source of their trade wealth, even more so than the Italians.
It's not like Italian weren't interested in the new world - so many navigators and explorers of it under the service of Spain and England (and i would assume France, though I dont know for sure) were Italians - so they had the technical capabilities, and people interested in doing it - they just had to operate through the main power dominating the peninsula at the time, Spain - or flock to it's arch nemesis, England.