For one thing, virtually every country on earth claims to be democratic, whereas only some claim to be socialist. There are many countries that claim the label of democratic that don't consider the DPRK to be a democracy, but the countries that claim the label of socialist, such as Cuba, generally recognize the DPRK as socialist. If would be strange to refer to a group of countries as socialist and then exclude a country that those countries recognize as being socialist.

It's worth noting that one of the main reasons the DPRK is not considered democratic is not because of the way the government and elections are structured, but because it doesn't allow its elections to be monitored by international observers.