The 1st page of releases.ubuntu.com seems to me to explain why:

Ubuntu Releases

The following releases of Ubuntu are available:

• Ubuntu 14.10 (Utopic Unicorn)
• Ubuntu 14.04.1 LTS (Trusty Tahr)
• Ubuntu 13.10 (Saucy Salamander)
• Ubuntu 13.04 (Raring Ringtail)
• Ubuntu 12.10 (Quantal Quetzal)
• Ubuntu 12.04.5 LTS (Precise Pangolin)
• Ubuntu 10.04.4 LTS (Lucid Lynx)

We are happy to provide hosting for the following projects via the cdimage server. While they are not commercially supported by Canonical, they receive full support from their communities.

• Edubuntu
• Kubuntu
• Lubuntu
• Mythbuntu
• Ubuntu GNOME
• UbuntuKylin
• UbuntuStudio
• Xubuntu

The cdimage server also hosts releases of other Ubuntu images not found on this server, such as builds for less popular architectures and other non-standard and unsupported images. For Ubuntu Desktop and Server on popular architectures, please see the links above instead.

• Unsupported Ubuntu Images

For old releases, see old-releases.ubuntu.com.

• releases.ubuntu.com

  Contains only current commercially supported releases images (i386 & amd64 only, 5 years period max LTS for server editions).

• old-releases.ubuntu.com/releases/

  Old releases images that no longer supported.

• cdimage.ubuntu.com/releases/

  Commercially unsupported favors and builds including alpha, beta and daily builds.

• cloud-images.ubuntu.com, old sub-domain is still working too uec-images.ubuntu.com

  Ubuntu Cloud Images are pre-installed disk images that have been customized by Ubuntu engineering to run on cloud-platforms such as Amazon EC2, Openstack, Windows and LXC.

  It was there in 2010 as shown in the history of UEC/Images Wiki page, Revision 2 as of 2010-08-19 20:59:56.

As canonical is a company that provides technical support, it should have should have such kind of separation.

Looking to the official links on www.ubuntu.com. They are all point to releases.ubuntu.com except Network installer and Ubuntu Kylin.

Another thing, they always try to minimize mirror size and mirroring daily builds just wasting of resources. Mirror script point only to releases.ubuntu.com

I couldn't find out any official note about such decision yet. But separation between releases and other builds back to 2005 at least. See Ubuntu Server Project Unleashed!. Even back to Oct 2004 with release of Ubuntu 4.10 which is the first release(just i386), Make a look on these few early mails from ubuntu-announce archive.

Cause they messed up the URL. It should be http://releases.ubuntu.com/releases/trusty/

What you are looking for is: http://releases.ubuntu.com/releases/trusty/ubuntu-14.04.1-server-amd64.iso