The average lifespan of a paper dollar bill is 18 months, whereas a coin lasts 25 years (source: usmint.gov).
By retiring the dollar bill and switching to the dollar coin (which is already in circulation), the government can save millions of dollars in printing costs.
Efforts to do this in the past have failed for the simple reason that the dollar bill continued to be printed. There was no reason to people to switch.
A secondary benefit will be the increase of jobs as older vending machines are upgraded to handle the new coins.