It's a rather high-risk, high-visibility announcement.
Surface 2 has only just launched so there hasn't been any trial. And no-one knows for sure if it will be certified by FAA, or how long that will take. If it fails, Delta will be looking very stupid.
Even if the Delta folks wanted to do SUrface 2, it would have been prudent to do some testing first. But I bet that Microsoft pushed them *very* hard for the announcement to be made now, to help with Surface rollout.