There must be somehing in it or Cessna, a company not renowned for jumping into new ideas and owned by the same group as Lycoming, would not be doing it!
Yes, there is and I think it's not by choice that they do it. I heard something about this a couple of months ago.
The story I heard, and it came from someone in the US, who was buying second hand C172s for flying school use in areas where avgas is unavailable and replacing the Lycoming engine with the Thierlert.
One of Cessna's dealers was buying brand new C 172's and doing the same thing. Cessna told them to stop doing it. The dealer, apparently Cessna's biggest distributer by a substantial margin, told Cessna where to get off. Cessna couldn't afford to loose this distributer and as a result have decided to fit the Thielert at the factory.