The answer is simple – target audience. I have been a .Mac subscriber since the day it was announced. I immediately grabbed my current .Mac email address and currently use it for iChat and for some email. Because my email address is so easy to guess it is inundated daily with so much spam that it is virtually useless for email — don’t bother emailing me there. Whenever I give someone my .Mac email address for iChat they immediately know that it is a “mac thing”. That immediately sets the target audience to just Mac users. By rebranding the product Apple can relaunch it and target the vast audience of users of iTunes. Those users can get all of the features of the new product line and not have a mac.
For me I plan to switch over to the new namespace when it becomes available next month even if it mean abandoning my current mac.com address.