Considering split screen co-op reduces the screen resolution to, say half by adding large black squares consuming a good portion of the screen. It could be a lot harder to implement even networked co-op, than it looks.
I mean assuming the code is 90-99% efficient, and you wanted to add netcode and further calculations for the actions of a co-op partner throughout the campaign whilst running at the regular resolution (720p), the addition online co-op would likely be impossible/involve a lot of hard work unless they were to compromise something like resolution or textures during co-operative play.
It would be nice to have though, of course. But at the same time I can understand why it isn't present. I'm sure they could add it but it just wouldn't be as easy as it seems, but regardless I hope they do. The splitscreen co-op is useless to me, not because I don't have anyone to play with but because the screen sizes are real small.
