Because it radically improves it. It does away with lesser-of-two-evils voting.The problem is time. Why complicate the process?
Republicans and Democrats both have been using it in primaries. At it's being adopted more and more for local and state elections.If Democrats would like to use that in a primary, more power to them.
What do mean? Of course they can. Lesser-of-two-evils is as much a problem there as it is in the Presidential race.In Congressional and Senate races, it makes no sense as third party candidates don't draw any real votes.