Could also be that mosquitos always managed to hitch a ride, but in so small numbers that they disappeared relatively quickly once they arrived, or it simply was too cold but some always appeared.

But now when it's a bit warmer, the same amount arrives but more survive.

In order to produce a breeding population, several mosquitoes need to hitch a ride on the same ship and stay close enough together that they can mate after arriving. They also need to survive the trip over without that much food. I imagine its quite unlikely.