If they don't come back you can't ask them why not. That's the point. It's a function of the level of competition. If it's a competitive location and you lose that customer, they're practically gone forever.
If you're collecting their email addresses to give them their receipts, this is probably not as problematic as you think.
On the other hand, I'd love it if the places to which I've never returned asked me why, and then changed as a result and invited me to try again.