So, it's entirely possible he simply chose not to ride.
He DID choose not to ride. He has stated it time and again for 37 years.
There's a long chapter that deals with that in Mes 50 victoires en 1973
**(his diary from the 1973). On a first note the journalist René Jacob (editor of Les Sports
, who made his timetable for the Hour record) reminds us that the Molteni team was one of the smallest team in number in 1973: 18 riders, some were very young. This means that for the whole season substitutions could hardly be possible. The spring classics + Vuelta + Giro, that's a span of 4 months. It was unthinkable to ride another GT. Merckx could have done it. His team mates, NO ! And I heard him say that again around 30 years later when commenting a race on TV.
So why did he choose to ride the Giro? Again according to what he reminds in the book that Mr Pietro Molteni was Italian. He chose to ride Midi Libre and Dauphiné libéré in 1971 skipping the Giro, which Mr Molteni accepted without any problem but he didn't dare to skip it again.
About the Vuelta he said that the first time its organizers asked for his participation was in Mulhouse 1971. He could already have done it in 1972. But in the meantime, Ocaña fell and the journalists (the French ones, mainly, as usual) badly wanted a revenge for 365 days. So Merckx did not want to be regarded as someone who avoids duels and accepted to give Ocaña a second chance. As he destroyed him in July 1972 he was now free to make the programme that he wanted and chose for the Vuelta.
And another think that we should bare in mind is that Merckx was NOT interested in records. He's not Federer. He's not Indurain. Even this year I have again heard him say "The only record that really matters is the Hour record".
What he however was interested in was diversity and winning as much different races as possible. In 1973 he raced and won the GP des Nations
because he never won it before. He raced and won Paris Brussels because he never won it before since it hadn't been held since 1966. In 1974 he decided to race the Tour of Switzerland and won it. He had never won it before. Etc etc etc.
**Coincidence: I've just bought the book last Saturday. The salesman told me: "And I think we can say that he was the greatest, can't we. I don't know if someone is ever gonna match him" My response: "No one will ever match him"