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 Group
You can find the
customer_group_id from the Admin URL, or by checking the
Find all customers in a particular group
SELECT * FROM `oc_customer` WHERE `customer_group_id`=1
Move all customers in one group to another group
UPDATE `oc_customer` SET `customer_group_id`=3 WHERE `customer_group_id`=1