Why don't they? Money.
All business decisions are fueled by money, that is the point of their existence. They want as much money as they can get, for a business to operating differently is a business design doomed for failure.
I'm sure their analyst have done more research then we can ever imagine to do.
A Mini with a dGPU would require virtually a completely different design then a model with an iGPU. So they likely simply looked at the demand (which we are a very poor representation of that btw) and said, the R&D, engineering, assembly, shipping, stocking, SKU vs the demand isn't worth it.
Then we need to ask if a dGPU is necessary for Apples (not ours, remember they are making it) intended use of the product. Someone sarcastically mentioned above "What do we need that much power for?!" but thats a true statement. Do you intend on doing professional grade photo/video editing on it? Because they have products for that. And really what are your plans for a Mini with dGPU? While more time consuming 4k editing can be done on it, hell I'll edit 4k video on my iPad Air 2 on occasion.
Obviously I agree a souped up Mini can/could cannibalize Apples other products. Again, more business decisions.
Point is we shouldn't be too upset with Apple, this is how businesses work they just want our money we aren't and will never be on the same side. Would I like a Mini with a quad core CPU and dGPU? Hell yeah! But I'd rather be surprised they make it then disappointed they didnt. Plus I'm a realist.