One reason: consistency. When you introduce new features, idioms shift and maintaining code that uses multiple idioms for similar purposes is simply less fun. In the worst case you make changes harder because you now have to make your changes compatible with more modes of usage.

Dependency (sometimes even binary dependency) with a library that doesn't support newer standard can be an issue. Not all compilers guarantee interoperability, and even with those that do there can be bugs.

An IT department that controls compiler upgrades, in my case :/. We're still stuck on not-quite-C++11 because we're still stuck on g++-4.3