This has been debated forever. So let met just hit some of the high point arguments and be a politician myself without taking a position.
1) No elected President - who prospered in the system of the year he was elected - has ANY incentive to change it. (This ought to be obvious to anyone).
2) A national primary is a terrible idea because it may well result in a nominee who has not been properly vetted and THEN a scandal is discovered from years ago - and the whole party and nominee are toast. In fact, a national primary guarantees that the guy with the most money WILL win (pretty much).
3) One proposal made by Walter Mondale some forty years ago would give us a regional primary. And guess what two things would happen? First, the person from that region would, in all probability, win the region, it would be like the Heisman voting. Secondly, the candidates would focus on the largest delegate prize exclusively. If Washington, Oregon, Utah, Nevada, Montana, Idaho, and California all have primaries the same day and all split proportionately......guess where the pols will maximize their efforts?
4) Of course then there's an obvious counter-argument.......the small states don't get any attention anyway with two notable exceptions, Iowa and New Hampshire. (Quick - name me one person who won the Idaho DEMOCRATIC caucus? You can't without looking, can you?).
(I'm at work - to be continued later....)