Why did Platinum take over as the default program for older versions of Sonar?

Well, the file extension - .cwp - is the same for all versions of Sonar. So in the way Windows works, each extension can only be associated with one application for double click opening.
 
You can get around this in a couple of stages, if you need to.
 
If you right click on a project, you'll get a menu, and that will have the option "open", but also the option "open with".
 
Different versions of Sonar might already be available in "open with", or they might not. But it's easy to add them. Give it a try, and report back here if it doesn't make sense to you.

Might be a setting somewhere, but mine always open with the version of Sonar in which it was last saved.

If you right-click on a .cwp file and go to "Open With>Choose Default program..." you are given a choice of programs.
 
If you click on (I am checking this in Win 8.1) "More Options" and then, at the bottom, "Look for another app on this PC", you can navigate to C:\Program Files\Cakewalk and set any of the SONARDPR.exe files of any of your installed Sonar versions as the default startup program, and the icon that appears for files with the .cwp extension will be of the default version you choose.
 
If you create, say, a text file, then change the extension from .txt to .cwp, the icon changes to your default version of SONAR.
 
If you double-click on the file, your default version opens it with an empty project (!) So Sonar knows nothing of the contents of the file.

Yeah, I have never known a Windows program to be able to differentiate versions in this way. As far as I understand it, at the moment of deciding which application to open after double clicking a file, the file extension is the only thing Windows knows about the file.