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.