I worked at Copper Mountain Ski Resort last year and was lucky enough to take part is the comped lesson program.
The Copper Mountain Ski and Ride School brochure publicizes all offerings made. Standard lessons, Private lessons and specialty programs.
The "Sunday Lesson Pass" was originally intended to increase participation (and thus revenue) in group lessons on Sundays. The pass was popular, as we can see by this thread, but it lost money on the whole because the people who purchased the pass used as many lessons as they could (which was their right!). As Tsavo said, higher level instructors get paid substantially more and this greatly impacts the profitability of a program like this one.
If the majority of Sunday Lesson passholders were folks skiing in black-level lessons, then it would have a tremendously negative impact on the profitability of the program.
If any company decides to modify or cancel an unprofitable offering -- well, that's just good business.
As a consumer, you're surely going to be upset that your skiing bargain has gone away ... but think of your good fortune for having had it in the past!