Because you can not delete a group if it has customers in it. And there is no easy way to move all customers to another group via the admin.
Warning: This customer group cannot be deleted as it is currently assigned to 2170 customers!
customer_group_id
number of a Customer GroupYou can find the customer_group_id
from the Admin URL, or by checking the oc_customer_group_description
table
1SELECT * FROM `oc_customer` WHERE `customer_group_id`=1
1UPDATE `oc_customer` SET `customer_group_id`=3 WHERE `customer_group_id`=1