Why does Google mail claim that Thunderbird is a 'less secure' mail client?

When adding another of my Gmail accounts to Thunderbird, it configures automatically, no problem. However, when attempting to retrieve email I get an 'invalid password' and a prompt to sign in to Gmail directly. This led me to Gmail requiring me to allow a 'less secure' app or desktop mail client in my settings. Is this a Google scare tactic or is Thunderbird truly 'less secure' by not using 'modern security standards?'