Had a bunch of alternate flight options offered in both App and online last Friday after a couple of flight delays (although I don't think it showed any alternate destination/origin cities -- but it was to/from SEA where alternates are not that close). I think either being below MCT or delays greater than 30 minutes may be the threshold. In my case, both flights initially had delays over 30 minutes, but the offers went away in one direction when delay was adjusted to exactly 30 minutes. The fact that DL has already rebooked you on new flights may be the issue here.