Because those two bills are most often used for fraud. It takes too much money and effort to make fake bills. 20s for instance would cost more to make than they are worth so the profit and risk is not worth it to the criminals who make fake money. Most people who would pay the DQ in fake bills are not aware they're fake. They sold a TV on Craigslist for example, and the cashier takes it without realizing too, so the DQ takes the loss in the end.
What April Sunshine said, with a splash of common sense. Unless you are doing a purchase that justifies the use of higher denominations, there is no readon to accept such bills. Not only is there the counterfeit risk, you also have the registers with only so much cash. If a cashier breaks a large bill, they limit thier ability to provide change to customers.
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