Why No 40% Silver 1965 To 1970 Quarters Or Dimes?

The mint wanted to remove ALL of the silver from the coinage. The 40% silver half dollars was a sop thrown to the western silver producing states in order to get their votes to approve the clad coinage. As originally proposed all the coins would have been clad and the silver states let it be known they would not vote for it. Without their votes it would not have passed. So it was agreed to keep the half, the largest coin, at least partially silver.

Thanks for the information guys.
I remember silver getting close to 50 dollars an ounce back then. I'll bet a lot of coins we'd all like to have now were melted to make some spending money.