My personal theory is that these three places are quite similar in terms of climate and these conditions are perfect for the virus to stay active for a longer time outside of the human host. All three (northern Italy, northern Iran, Wuhan) at this time of year are dry, cold at night and cool to mild in the day, a classic Mediterranean winter climate. Places that are hot and humid, or cold and wet - or indeed very cold - are not seeing the same level of spread. Its quite obviously not spreading in tropical regions, like Thailand, Vietnam and Singapore that still have a low number of cases despite being there a long time. It also hasn't taken off in Canada, Russia or Scandanavia. In Europe, outside of Italy, it is spreading quickest in Spain and France, and there will be parts of the US that have very similar climates. Thats where I think it will be most difficult to stop
Just my own little hypothesis
We've gone from 94 this Thursday to 203 today in Sweden, would not surprise if it goes to thousands soon enough..