Why is the Aran class so unpopular?

I think you just basically answered your own question. Kaiser has most of the same appeal of an Aran with more bells and whistles, better numbers, and some flexibility in attacking methods. About the only thing Aran has going for it over Kaiser is a more dependable stance and a level 200 mount.

As a fellow Aran lover, I have to admit that Kaiser's combo system was more well designed in comparison to Aran's.