Because yum does not support Python3.
You can run command vi /usr/bin/yum, change /usr/bin/python to /usr/bin/python2 in first line.
Thus you can run the command yum by Python2 instead of Python3.
Note however that this will make your setup unsupported and thus unmaintainable (as does what you did). You will likely have other similar problems in the future with other system packages.
If you want to use an alternative Python installation, consider installing it into /usr/local, /opt or using pyenv.