I believe Pro Tools started as a solution for replacing the tape machine in a studio.  Engineers could continue working with their familiar consoles and outboard gear and record to a computer with greater editing capabilities and potentially superior sound.
 
Sonar started as a MIDI sequencer and did not add audio until later.
 
Both DAWs have grown to do everything but I think the history explains why Pro Tools got the lead in large facilities, which in turn gave it an aura of being the professional choice (whether justified or not).
 
I greatly prefer Sonar and since I am a writer and arranger, it has a better workflow for me.  Sonar also bundles a lot more bang for the buck.  You could argue all day about little features that are better in one than the other—and people have done that here—but it's like arguing Honda v Nissan.

PT is similar to Apple in that their marketing strategies early on targeted business and schools appropriately. At those times colleges were also able to throw money at things, and students want the same after graduation since they are familiar with the product. Once a foothold is gained there, it is easier to progress.

The ultimate example of "target the young" is Disney, since it sets up a repeating cycle for each generation. No parent who grew up on Disney could ever deny their children.