You can read more about why Drupal Commerce exists here:
I believe there was a previous post on that site before which explains more throughly, perhaps it's in these forums.
Ubercart has served it's purpose over the years, but if you try and develop something robust with it, you'll run into nuances. IMHO, the only way to fix, is to take what's been learned over the years and start from scratch...and I believe that's what's going on with Drupal Commerce and I look forward to it's future releases under D7.
Ubercart has been ported to Drupal 7 as well, so you're welcome to use that should you like. For those looking to become early adopters, it might be a better idea to go that route, as Ubercart is (i assume) much more robust than Drupal Commerce at this point.
What I would like to see is a stable release of Ubercart under D7 with bug fixes only and focus the future features and development on Drupal Commerce...but that will depend on those who want to donate their time of course. I do think fractioning the community would be a bad idea in the long run.