why is it that....

[Aquattro]

It is because the vast majority of people buying/using/stealing Ca reactors keep SPS. It's generally accepted that they're the only ones nuts enough to spend that kind of money on coral growing toys.
Also, I don't believe that a tank of leathers consumes calcium nearly as fast as a SPS tank does, therefore presenting more cost effective measures of Ca supplementation. I could be wrong, but I'll need to see a bleached leather skeleton and compare it to a bleached acro skeleton. I think the latter contains more Ca.