Because antivirus software is constantly updated.
When this offer ends, you could try another 6 months offer, by another antivirus softwarehouse.
By the way:
– Windows 10 defender and firewall are free.
– ZoneAlarm antivirus and firewall are free.
– Comodo internet security is free.
– AVG antivirus is free.
– AVAST antivirus is free.
– Avira antivirus is free.
– Bitdefender antivirus free exists too.
– And so forth.