In the mid-80's, there were tremendous advances in the field of exercise physiology. It was discovered that a structured training program could build a rider to a 'peak' of fitness that lasted for only a couple of weeks.
Prior to the mid-80's, cyclists generally rode a lot of long slow miles in the winter and the found their 'race form' for the classics and worked to maintain it throughout the season. Sure, they would rest prior to and following stage races and take it easy for a couple of days prior to marquis events such as World's or spring and fall classics, but the science of a year-long campaign to reach optimal form for a short window of time was not well understood.
Enter Greg LeMond. LeMond and his training staff recognized that a rider could be at his absolute best for only a couple of weeks out of the year. Given that the TdF was the only race that had a decent sized following in the United States, Lemond decided to have that be the sole focus of his season for obvious financial reasons. He trained with near singular focus on the Tour and achieved fantastic results. He built to a second 'peak' for World's and showed success there, too.
A recent article in Cycle Sport shows Fignon racing tough in an edition of Paris-Roubaix during a year that he also waged a campaign to beat LeMond and Hinault in the TdF. If a rider attempts to have peak form for both events, he will compromise his chances of success in either if his competitors have singular focus in their training programs for one major event.
Since LeMond's training program and the success it brought, the racing programs of cyclists have changed dramatically.
As dominant as Merckx was, I wonder how he would have done if each of his rivals had focused on one event in their entire year's preparations while he tried to dominate from March until October.
There are currently very few riders who show good form all season long. None of them are top GC contenders or Classics specialists. Jens Voigt comes to mind as a year-rounder, but most of his victories come at times in the calender that fall between the Spring Classics and the GT's or after the GT's when the World's contenders are focused on the rainbows.
Sprinters have better successes throughout the season in part becuase the discipline is as much about tactics, teamwork, and skill as they are about pure power production.
I the past decade, the training programs have been further refined and programs increasingly tailored to single events. Armstrong's run of 7 tours would not likely have occured if he had tried to maintain prowess in the Spring Classics or attempt a second serious bid at a world championship. Armstrong, Charmichael, and Bruyneal refined the science of building to a specific 'peak' for the TdF beyond what any other team could accomplish.