Why don't you travel around and ask about Ulfric. It soon becomes clear why he really fights.
One thing I love about TES in general and Skyrim in particular is how unreliable your sources of information tend to be. Like in real-life, you can only truly judge a man for yourself. Yes, it's clear he doesn't have much love for elves, but while Ulfric's prejudice is mild compared to the empire, which actively kidnaps, tortures, and executes people based on who they worship in the privacy of their own home... not just the guys that openly preach the divinty of Talos, they have you go into people's houses to search for hidden Talos amulets so they can arrest them. Ulfric, on the other hand, simply allocates his resources on a nords-first basis, allowing the elves in his territory to live their lives how they'd like, he just doesn't offer them anything in the way of public support. Hell, the empire is willing to execute you, the player character, just for being someplace
near where Ulfric was at... and they're the ones you want to throw your support behind?
SPOILER... As for charges of being power hungry, when you win the civil war in his favor, the first thing he does is announce that he will not be taking the title of high-king for himself, as it was assumed he would do, he announces that he will await the decision of the moot. He could have easily taken power then, but he chose to allow the democratic process a chance to work. It was a political calculation, to be sure, but it's not the move of a man who just wants power, it's the move of a man who wants to rule over a healed and unified land.