Did you by any chance by an "E" fare economy ticket? This is a new fare class that Delta introduced earlier in the year that does not allow you to select a seat until check in.

I don't know how this would show up when you go to select seats but it would be something to check. If you go into trip details, you can pull up the receipt for your ticket and you can check the listed class for your ticket to see if it is "E" or one of the different coach fares from the long alphabet of available fares.