I think Bipolar 1 is looked at as more severe than 2 because of the manic episodes. Meaning they can be so bad and disruptive in so many ways, especially when someone gets pyschotic.
Being that depression seems to last longer and maybe more severe in type 2 than 1 is just as serious and dangerous- but can be quiet, not always outwardly seen (intovert) as apposed to mania- which is "in your face" and loud. extrovert

I think of it like the mania is more objective and depression subjective. Both are equally serious and severe I guess.