The vast majority of Windows users don't need all the features that the Ultimate/Enterprise editions offer. I think the reason most get Ultimate is because of it's name, period. "ULTIMATE" the best, greatest, superb, stupendous, exciting, whatever. Actually to be honest, most users would be satisfied with Home Premium.
Pricing wise there is not that big of a difference between the Professional and Ultimate editions. $40 for the OEM verisons, $17.50 for the Full retail versions, and
$15 for the Upgrade versions. These prices are the current ones at Newegg today.
There is an even mix of the product line both at Newegg and Amazon. Probably two of the bigger e-tailers on the web.
I like to compare it to the great dreadnought battleships of the early 1900's when the major naval powers were trying to out build the other countries in ship size, tonnage and gun size. It's kinda like, "my thing is bigger than your thing".
I feel its a great marketing ploy by MS because the name Ultimate appeals to the ego-centric area of purchasing decisions.
I personally use the Pro version because it has what I want and saved me money in the long run.
This all my opinion naturally, however I would like to see a breakdown of the sales numbers of each edition. Most of the pre-installed systems from Dell, HP, Lenovo, Acer, etc., are the Home Premium edition.