COVID-19 is being regarded by the WHO and public health bodies around the world as more serious than the other viruses mentioned because:
it is both very infectious and with a comparatively high death rate
The current mortality rate of COVID-19 even if it is "only" 1-3% in Europe
is far higher than the 0.01-0.1% for seasonal flu, which causes 600-10,000 deaths annually in the Uk
and the 0.026% for swine flu
Chickenpox is several orders of magnitude lower in danger:
only about 25 people per year die in the UK of chickenpox, which is a widespread disease
So "chickenpox parties" would be batshit (and would be irresponsible even for chicken pox)