Because polar vortex storms aren’t a very common phenomenon. Aside from the past couple of years, the previous occurrences seem to be 2014-2015, 2013-2014, and then 1985 prior to that.
https://en.m.wikipedia.org/wiki/Polar_vortex
So, we have a very rare, very extreme weather phenomenon that’s hit multiple in the last 10 years after decades of static behavior. The wiki on the Polar Vortex even attributes the 2013 storm with popularizing the term due to the heavy reporting.
This isn't a general-winter-weather thing like that. The polar vortex is a very particular phenomena that only lasts 1-3 days* and doesn't necessarily come with snow. Its signature is just extreme cold, on the order of -40 degrees or lower, even in areas that only very rarely go below -5 F (-20 C).
* technically it's always there around the north pole, but we only talk about it when it dips down into populated areas.
So, we have a very rare, very extreme weather phenomenon that’s hit multiple in the last 10 years after decades of static behavior. The wiki on the Polar Vortex even attributes the 2013 storm with popularizing the term due to the heavy reporting.