First of all, yeah sure cross-platform play is nice and all...but lets go from why it might not happen or does happen but takes longer.
DE is Full ahead focusing on the next few major updates, focusing all power to that so they cannot stop to make it cross-platform play. As that will take time away from updates. Not to mention that if we have cross-platform play, you do realize that updates will be increasingly slower since they need to make a patch and updates for all gaming consoles and pc to be able to play together as soon as an update is launched so nothing conflicts.. DE are thinking about cross-platform data transfer again and that is something that might be easier to do than doing something like a full cross-platform.
ALSO i AM not against cross platform, I think its awesome idea..but how warframe is, I don't think it should happen.