I know it's fun to speculate, but the "why" doesn't matter. Quarters produce the least. Asking why won't change anything.
But here's a strategy you can use to improve your quarter averages. Buy bags of dimes. They're the lowest-valued coin with 90% silver. They don't fill up as fast as quarters. Silver averages are better. If you hit a huge collection dump of dimes at a certain bank, buy bags of all the denominations and hope you get a multi-denomination collection dump. If part of the collection dump is quarters, you will dramatically increase your average silver per dollar amount searched. Search less, find more.
There is the chance you miss the quarter collection dump because quarter bags fill up faster and could be shipped out before the dime bag even fills up. If you have the time and money, buy all bags of all denominations. That way you won't miss anything.