- The operating system used to deploy your applications
- The availability of the libraries your application used
One extreme example of how the operating system influences this decision is when CentOS is used. CentOS is really close to Red Hat Enterprise Linux (RHEL) minus the commercial support, and many companies that started off with RHEL and grew internal teams, ended up moving to CentOS. There are a lot of good reasons to use CentOS. This Linux distribution is popular and based on a robust set of management tools.
However, using CentOS means you cannot use the latest Python version for your projects unless you install a custom Python instance on the system. Moreover, that is often considered to be bad practice from an Ops point of view because you go out of the supported versions. For that reason, some developers were forced to use 2.6 for a very long time, and that prevented them from using the newest Python syntax and features.
The other reason people stayed on Python...