Why is chromium-browser still at version 53 for Ubuntu 14.04 Trusty Tahr?

At this moment, Ubuntu 14.04 only offers version 53 of Chromium (released at 2016-11-16), while Chrome version 56 was just released. More recent versions of Ubuntu, e.g. 16.04, are already offering Chromium version 55 (and I guess 56 will follow soon). Looking at the upstream project, chromium-browser, I see that they don't offer a newer version than 53 for Ubuntu 14.04 either.

Is there a reason why a more recent version is not available? Has chromium-browser stopped supporting Ubuntu 14.04?

From the staging PPA for Chromium, it seems the builds are failing for some reason. The packagers maybe aware of it, a bug has been filed. You can mark it as affecting you.