How many failed dollar coins has the US had?
The small sized dollar coin failures include the Susan B. Anthony first minted in 1979 (hideous design, but consider the subject) and the Sacajawea (begun at the time of the State quarters program). The typical dollar note lasts 18 months in circulation till it wears out, but a coin will last decades. The coins have some popularity with collectors, but don't really circulate. One of the proposed uses for the coin would have been in the vending machine industry. They didn't want to get on board with the coin, because of the adjustments needed to their machines.
It seems change (ha, ha) is something folks don't like.
I suppose if the gov't wants to save money, it will have to look elsewhere.