Well if you look at the factions goals, playing a non-Nord would be like fighting against yourself. In the end, you would have to kick yourself out of the country you just liberated. That would just be awkward.
The Stormcloacks are for an independent Skyrim, but has it been said that they are also racist sh*t?
I don't see why they should want to kick my Khajjit out of the country as long as I don't support the Empire and help build the Skyrim nation. If they really are just a bunch of racists then it will be an easy choice, atleast the Empire has the same rights for all.