Romania's proximity to both Constantinople, and in later years, the Russians, put pressure on them to be Orthodox. But, I agree with Janprimus that longer, earlier and more complete Hungarian dominance over the country could make it Catholic.
Then how is this achieved?
Well, I'm not exactly an expert on this, but maybe a weaker Turkish presence in the Northern Balkans, allowing the Hungarians to swoop in and take Romania fairly early on?