The pro touring trend is all about taking the wonderful styling and massive motors of the muscle cars and putting newly engineered suspensions that handle like a super car, but look as awesome as only a muscle car can. The large diameter wheels and short sidewalls are all about cornering. Obviously there's a limit, but we're talking about pro-touring, not donks or high-rollers, which is all about looks
Pro-touring is more about handling and cornering. The short sidewalls allow for better cornering and the larger diameter wheels keep the overall outer diameter roughly the same as the smaller diameter wheels with taller sidewalls.
By the way - here's my 69 GTO Pro Tourer: