You're onto more than you think. Cannondales were the run-of-the-mill bike about 10 years ago. It seemed that everyone owned one, road, velodrome AND mtb. People made fun of them all the time, "Crack 'n fail" was their nickname. Now Trek has taken the market and is now the target.

Another possible reason is that Treks seem to be love 'em or hate 'em bikes. I don't know many people in the middle ground, which is probably due to their unusual geometry which people either find comfortable or impossible to fit properly. I don't like Trek carbon frames because they flexed way more than they ever should have under my 120 lb body (back then, not now ). However, it did fit quite well with my long arms and short legs. I just couldn't handle the swaying BB shell on climbs and the fork looking like it was rubber on the descents.