A few reasons:
The main reason is because we wanted to distinguish it from our stock firmware. Until we were comfortable with getting a Matrix version approved by the community, we wanted our users to use with caution, and understand that it's far from a Mass Production level release.
Another reason was because of the Site makeover we had in the works. If you notice, we started hosting other firmware for other products in this manner too.
Some of these firmware versions will eventually go to our website and some (such as Alpha) will remain hosted through these 3rd party servers.