Sony has very few professional sports/nature/wildlife shooters who would pay for an expensive fast 500mm. Also SONY is very timid when it comes to their investment/commitment to optics for their camera systems. They are hedging their bets and have not invested in optics design, manufacturing and marketing to the same degree as Panasonic, Samsung or Canon/Nikon. This is a shame.
I expect it's the bean counters that are holding it up. Not sure they can make a profit off it, and they are making most of the benefits with the implication it's part of the lineup just showing it without having to spend the money to produce it.
I agree, Sony is very bad about thinking of interchangable lens cameras as a system. I believe they are still considering this system to be a throw away camera and lens. One can see it in this, in their pretty poor service system, in what they have not supported that Minolta did.
It's not just the pros who buy the big lenses. Though it's certainly true few can afford them. Even pros are more and more just renting them when they have a job that needs them.