Simple. With side inlets, you are going to be in the way of the air flow. So that's dangerous to people blocking the airflow.
India News: In a freak accident, an Air India service engineer died after he got sucked into the live engine of an A-319 aircraft at the Mumbai airport on Wednesd
timesofindia.indiatimes.com
With inlets in the nose, no one's blocking the airflow, so the ground crew can start the engine long before the pilot is strapped in. That saves a lot of time. So the pilot can taxi out as soon as he is strapped in.