It took many years to have this spread across the country. One only has to look at the proliferation of non native invasive species of plants and animals into areas where they did not originate. We have mitigation measures to control rabies because it can spread rapidly once it hits a certain level. There is a point when linear growth turns exponential when those limiting factors you speak of are not enough.
Vector borne pathogens often do not make the host vector sick. That in large part is what allows spread.
The limiting factor is the lifespan of the host, though. Many of the animals described as hosts here in North America have fairly short lifespans.
It took many years to have this spread across the country. One only has to look at the proliferation of non native invasive species of plants and animals into areas where they did not originate. We have mitigation measures to control rabies because it can spread rapidly once it hits a certain level. There is a point when linear growth turns exponential when those limiting factors you speak of are not enough.