EXPECTATIONS WERE fulfilled -- extravagantly -- in Tuesday's primaries in the South. That frees up the chorus for at least one week from its regular duty of explaining on Thursday why Wednesday's news of Tuesday's choices was so dramatically different from what had been expected on Monday. This, in other words, is a week off -- so we thought we'd take the opportunity to vent a general observation.

It concerns the Big Picture. In both parties there is now a strong front-runner who is regarded by many in that party as being either not up to the job of president or vulnerable as a candidate for it or just plain wrong on the issues. Mr. Reagan and President Carter have this much at least in common and one other thing as well. That other thing is that each is relatively well protected against entry into the race of new opponents whose challenges might be stronger than those of the already fallen and staggering competition. It's not going to be easy for Gerald Ford, for instance, to get into the race in a significant way at this date -- and we remind you that the date is only mid-March.

The former president, of course, might be able to bring it off, just as it might (though less plausibly) be possible for someone other than Sen. Kennedy and Gov. Brown to plunge into the Democratic nomination contest. We are not about to argue that Mr. Carter and former governor Reagan should be defeated or that other candidates lurking in the wings would be preferable to them -- only that the "reformed" process by which the parties nominate their candidates has closed off an important opportunity. When you ask why neither Mr. Reagan nor Mr. Carter is likely to have to deal with new, tougher, late-entering competition, the answer is simply that multiple primaries with closed-off filing dates and the arcane provisions of financing have made that very difficult.

To some extent current law and current party rules that make this so reflect a reaction to an earlier situation in which party hierarchs could, at the last minute, meet and arrange to supplant some poor, worn-out candidate who had had the decency to take his candidacy on the road and put it to the test for months before. But the thing looks to us overregulated now in the other direction. Surely at some point the parties and -- to the extent that it is their doing -- the Congress and the states ought to try to put back into the system some of the possiblity for change, surprise and adaptation to moving moods and events. This is another area in which the reformation appears to have gone too far.