I've never seen these.  Happy not to.  They look disgusting.

I would say like a lot of introduced species, they appear in an area and then multiply prolifically as they have few natural predators.

They blurb on them SAYS they are susceptible to cold, so warming temperatures will help them to survive winter and therefor have a larger starting population for multiplication in spring/says.  

We keep flies out of our house with ripcord and fly screens.  We are close to a sheep farm so in the absence of those we get lots of flies.