Why no retina screen available on MacBook Air?

Because Apple didn't make one.

Duh, we don't know. We aren't Apple. If and when Apple decides to make a Retina MBA they will announce it. Then you will know and so will we.

To amplify Kappy's remarks, you should check out the Apple Support Communities Terns of Use:

Apple Support Communities Use Agreement

We are not allowed to speculate on this forum about why Apple did or did not do something.