I do not think the main cause of the disappointing sales so far is that this game went multiplatform.
It is the timing. If Tekken 6 was released a year or two earlier, it would have less competition and probably done much better if not well.
Unfortunately, expectations have raised quite a bit due to Street Fighter 4 and Blazblue: Calamity Trigger. It also does not help that both of these titles were released this year and have sequels/expansions already lined up.
Perhaps Tekken 6 would have done better if Namco polished the system or added extra incentives but it looks like they did not. How are you supposed to impress or attract reviewers and potential buyers like this?