My thinking behind why they kept it on PS2:
1. Xbox is releasing the 360 this month, it would be retarded to have it on the regular Xbox with a month of sales before the 360 comes out. And it maight not even work on the 360, since they won't be fully reverse compatible.
2. The 360 is an iffy system. More people but Sony than Microsoft, so it is more likely that they will have less sales going with Microsoft than Sony. Also there are the many qualms with it being a launch game, because of the DOA series.
3. Even though the Gamecube had the highest sales of the systems the majority of those sales were because of Link being a character. Without Link, I bet they would have lost half of thier profits.
4. So by process of elimination, it falls on Sony. While it is the oldest system, it is still the most popular. Even with the crappy control layout, people continue to buy it. And plus, every tournament I ever went to used PS2's as the machines that ran them.