I have seen that the default standard in IDEs is usually not the newest released standard, not even the newest standard in the IDE. For example JetBrains' Clion has C++20 and C++17 but the default option is C++14.
Is there a reason not to use the newest released standard?
As a general rule, use the latest standard if you can.
But, there are some reasons why you may in some situations choose to use an older one.
Your code makes use of features that changed behaviour in newer standards or were removed outright. If you don't have time to update your code, compiling for the older standard is reasonable.
Your tool-chain may not implement the new standard correctly. There could be known bugs that force you to stick to an older one.
You need to support multiple compilers on multiple platforms and not all combinations support the new standard yet.
You need to be binary compatible with code built by an older compiler for an older standard and you don't have the source to recompile it. In that case you may be forced to use the same old compiler and language standard to ensure ABI compatibility.
Internal company politics may mandate a specific version for arbitrary reasons.
Certification requirements may mandate use of a specific compiler and/or language version.
Familiarity with the new features may be low on your team, so using them may increase the risk of bugs.
Etc (I've seen all of the above happen in real life btw)..
