The Mirrorless camera market has grown in recent years, especially with developments from Sony of their E mount camera system. This has resulted in a decline in SLR camera sales for the Sony A mount camera system, making it uneconomic to creating new products for this system.
The future of lens design for the Sony E mount mirrorless cameras however is buoyant, which is why Tamron released the Tamron AF 28-75mm F/2.8 and a 17-28mm for this camera system.
In the future, even more lenses will be developed for the Sony E mount.