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.
No plans to move ever, or just sticking to it for a while? The latter is perfectly reasonable, the former seems a great way to end up with technical debt: at some point compilers might (although it is admittedly very unlikely at least for gcc) drop support for older standards, 3rd party libraries as well(this is much more likely).
