Traditional antivirus is reactive. You are paying for the mistake after it has happened. MDR actively looks for hidden attackers, sets traps for attackers to reveal themselves, filters out the noise, and contains then mediates the attack. All reporting to the organizational manager the severity of what is going on in real time.
Traditional Analogy:
Antivirus = smoke detector, tells you what happened, but up to you to put out the fire.
MDR = Fire Alarm company cooperating with local fire department to not only alert and confirm the smoke detection, but respond to the incident in a timely fashion so the small smoldering fire does not become a damaging fire.