I'm gonna guess because they were anticipating the ActionResult to gain methods and properties over the life of the CTP/beta. If it was an interface, every change to IActionResult would break existing code. Adding another method to the abstract base class wouldn't cause any problems.
Interfaces are great for allowing a class to implement multiple contracts, such as when you know that a type must be two different things. In some cases, this can encourage creating a type that has too many responsibilities.
Action results have a single responsibility and it didn't seem like there would be any scenario where you need an object to be both an action result and something else. Even if you did, it's possible to do via composition. So in this case, we went with ABS to allow us greater flexibility after we RTM to make changes if necessary.
However, if there's a specific scenario we're blocking in which an interface would be preferable, we'll consider it. We can always do it later in a manner that's not breaking.
You can even do it yourself by writing your own action invoker, which only requires you to implement IActionInvoker (an interface) and that invoker could check for your own IActionResult rather than ActionResult.