For the reason of differentiating tenths of a Dollar. For if we used periods instead of commas, how would one write $1,500.53? Does "$1.500.53" look nice?
Equally as interesting, I see many Europeans who use a common in place of a period, such as $500,53 instead of $500.53 as we write in the USA (and Japan). But how then does one write a longer figure? Like this: $1,500,53? Where 53 is the change?
Perhaps I am just biased, but I like the use of a comma and period as we normally use in the American numerical system. It just makes more logical sense and avoids confusion.