Lenses can take a long time to go from design to manufacture, and the A7 line isn't all that old. The success of it seems to have taken most people by surprise. If they knew what they knew now, they'd probably have started designing some a long time ago. Unlike going from say Canon to Nikon or A mount, E mount has its own set of design requirements so they can't really just stick a new mount on an old lens. New designs take time, and they started late. I bet we see at least one Sigma or Tamron lens this year though.
Sony make the E mount specs available to any manufacturer royalty free - it's why we saw the old 19,30, and 60mm lenses from Sigma (and I think a Tamron or two?) after all. I'd be surprised if they've gone back on that decision and closed things up.