I'm guessing that Anaconda gives you both 2 and 3 and that when NEURON detects that you have 3 installed it assumes that you want to use the more modern version??
I have no personal experience on this; A student reports:
having trouble getting my Python 2.7 Anaconda env working
...
py2.7 problem turned out nothing to do with macos. for whatever reason, Neuron install added following to bash:‘code’
export PYTHONHOME=“/anaconda3” #added by NEURON installer