There will never again be updates to TeX Live 2015 or earlier. Soon, TeX Live 2016 will be available and that version will then get updates, so you will get updates if you install the new version.
There is an annual period during which the previous version of TeX Live is frozen and the new one is tested. Eventually, the new one is released and becomes the current version. At that point, users can install the new edition of TeX Live. That new version's
tlmgr (the utility underlying TeX Live Utility) can then get updates for the new installation until it, in its turn, is frozen forever the following year.
Thus the cycle continues unto somewhere in the vicinity of eternity.