With the personal life insurance, you choose the amount of insurance. Usually, people take a benefit amount larger than the mortgage balance, so they can cover other risks too. With most products, the death benefit doesn’t change over time, it remains level.
With the mortgage life insurance, you are insured only for the remaining balance of the mortgage, not a penny more. Since payments on the mortgage are made regularly, the mortgage balance decreases over time and with it, the benefit of your insurance decreases too.