Rob, perception is reality.
It actually doesn't take very much research. You can literally compare the amount of requests that specifically ask for CPanel, over the requests that specifically ask for Ensim, H-Sphere, or Plesk at most of the hosting forums.
Whether this is fueled by the fact that most hosting companies (not all) offer CPanel, due to it's low cost, or the fact that it's included with many of the predefined dedicated server packages out there, is inconsequential really. The customers may have been introduced to CPanel because of this, but they learn it, and ultimately request it more than any other.
Again however, it comes down to what the majority of each company's customers want.
It's almost the same with browsers, and your hosting site. If you cater to or the majority of your clients are developers or designers, you can bet your site is being viewed in Mozilla, Opera, Mom & Pop's Happy Internet Thinga Majigger, and any other browser they happen to find along the way, while the majority of average internet users (home, and average business), will never see your site with anything other than IE. This is generally due to the fact that most of their systems only come with IE, and they've never found a reason to get anything else. Again, this does not account for ALL average users, but most indeed.
In other words each business has to make it's own mind up !
You are indeed accurate. The point is, datacenters have made it much easier for each business to make their mind up, by including certain control panels in the package, and most of the time it's CPanel, which again, may or may not be fueling the general overall demand for the CPanel control panel.