The problem is that they're such different business models. If MS makes one SKU for Windows, they would basically have to keep that price at the Home Premium version or risk losing the consumer sales. That would result in in significant revenue losses from the many enterprise customers and OEM's that go to professional/ultimate/enterprise.
To be fair to MS, while you can no doubt obtain a product chart showing a dizzying array of Windows versions, for Windows7 consumers there were basically two choices - Home, and Business, with Home now not being nearly as neutered as Vista home was (the move to higher versions being supersets or the versions before it was finally done, well overdue).
It will be interesting to see if they revise the Starter versions with Windows 8 - netbooks in late 2012 will be quite capable machines compared to when they first hit the market. They certainly can't ship it without Aero or the new Start Menu in that case, it's such a significant part of the experience.
