Price, games, first to market, mainstream appeal, existing brand recognition. These are the key factors to being a market leader. Wii U launched early, so it checks one box. Right now it has a weak game library and is the most expensive. This fall it could swing to being a great middle ground between PS360 and PS420 in the eye of a lot of consumers. Nintendo still has about 50% of the game market worldwide, so it still has a lot of positive brand recognition.
If Wii U is the cheapest and if it has exciting software, there is a very good chance of it being a market leader.