It is by design!
Upgrades between LTS releases are not enabled by default until the first point release, 14.04.1, scheduled for July. It is recommended that most LTS users wait until then before upgrading to 14.04. You could add a '-d' flag if you're in a hurry!
Re:Ubuntu 14.04 released
by mrinterweb on Thursday April 17, 2014 @03:53PM
(#7605897)
I used the '-d' flag and the release that it showed was the development release. I tried to proceed with the 14.04 development upgrade, but the upgrade failed.
Re:Ubuntu 14.04 released
by tyleregeto on Thursday April 17, 2014 @02:20PM
(#7605327)
I'm not seeing it either, its not just you. Probably still propagating or something. Hopefully soon!
As a friendly reminder, if you want to download the release as quickly as possible, use the torrent from http://www.ubuntu.com/download/alternative-downloads and be sure to seed for others.