Because python3 and python2 are incompatible dialects, you have to say which one you want, and there's no reliable way to use just "python" and no useful reason to have it on $PATH. Any code needs to declare all its dependencies, including either python3 or python2. If I didn't want any specific dependencies, I'd be targeting POSIX and writing in Bourne shell.
If you mean OS distros then this is not a concern. Unless your python needs are very small then you shouldnt use the OS distro for your python apps you should be running them in independent virtual environment with independent versions and packages