Why can't I submit portals any longer?

NIA has disabled it temporarily, because they are swamped.

See this post: https://plus.google.com/+NIAOps/posts/HfrZxjvjjdU

Niantic (if you follow the link in Maerlyn's answer) says they are reworking the portal submission and edit approval system.

We can expect some kind of voting system allowing players taking part in the approval, but it's not easy to implement without allowing factions to consider it as a sub-game where you create portals useful for you and remove the one used by opponents.

They are much less overwhelmed by portal submissions since they have disabled the seer medal. Submission of good quality portals in place where portal density is low were often approved in a few days.