This is what I'd think why:
1. Negative hype from the first (good game but not something people wanted to pay $60 for)
2. Competition from games releasing after and before it (mainly Mass Effect 2)
3. Lost in the market (although commercial advertising was good, no one really knew about it til after it was out as it got lost through the epic holiday season we had which is one of the issues surrounding January release that aren't necessarily a big name).