Well, there is some exciting stuff they can run with, it's true... but... well.
Korean war? Too sensitive. Technically, it never ended.
WW1? Too hard to sell. Americans were a very minor player in it, and they make up the biggest slice of the market.
WW2 again? Unimaginative. It's been overdone; too soon for another one.
Modern day? Conflicts too directly with the MW series.
Far future? Far fetched, out of touch with the COD series itself.
They prolly spent a long time throwing darts on the board in the design office trying to come up with other ideas, and frankly... got nudda.
The next COD game is going to rehash an old war they've done before through sheer lack of alternatives - probably WW2. That's my guess.
Cor Blimey, guv'na!