The switch to GCC 5 for Ubuntu 15.10 was announced a while ago, and now Ubuntu developer Matthias Klose has also put a clock on it. It's going to be a difficult transition, but it needs to be made.
One of the issues with a new version of GCC is that packages need to be recompiled with the new version, and that can be a hassle, not to mention that it can also generate some issues that can't be foreseen. This is the reason the new GCC 5 packages will be made available via a PPA, which is a lot less intrusive than making them a part of the OS all of a sudden. If problems do arise, it's much easier to roll back those changes.
Build failures with GCC are an important aspect and the biggest problem. This is why upgrades for GCC are no made all that often, as too many things can go wrong, even if GCC updates are released all the time.
GCC 5 to land by the end of the month
The adoption of GCC 5.x is an important milestone for the Ubuntu 15.10 (Wily Werewolf) development cycle, and it's likely that things won't be quiet after it happens.
"We are currently preparing the switch to GCC 5 as the default compiler for 15.10 (wily). Unlike earlier updates to newer compiler versions, which only required updating packages to newer language standards, this time we have a partial ABI transition in the standard C++ library (libstdc++6). Basically we'll have to rebuild around 3500 source packages, and will have some hundred follow-up library transitions," wrote Matthias Klose on the official mailing list.
It's worth noting that the Debian project is doing the same thing, and the move to GCC 5 is also happening there at the same speed.