My guess is that they want to get the 5.0.1 update out fast and so they didn't want to waste any time making sure their first OTA update goes well. They've probably planned out the 5.1 update as their first OTA (with scheduled roll-outs, monitoring teams, contingency staff and backup plans, etc.) and they don't want to change any of those plans which would potentially slow down the release of 5.0.1.
Just a guess. Who knows.
Or maybe it just takes longer to show up in the OTA system. We've never done this before so we just don't know how it's going to work.
EDIT: Starting to see reports of OTA working, so maybe my second guess is all it is.