Answer:
Interest rates are generally very high
Explanation:
Installment plans consist in paying small amount of money over regular intervals of time (the most common is monthly payments) to cover the total amount of debt, this payments are usually charged with interest rates (percentages of the cost of borrowing money), this is how the lender make a profit.
Because interest rates are generally very high, people end up paying much more than what they borrowed or as double of the original cost of what they bought.