There are two possible reasons.
1. The man feels insecure or uncomfortable about earning less
2. The woman feels used or resents him for earning less!
In both scenarios the couple's mindset has gone from:
"Us & We" to "You & Me".
(They're not viewing their marriage as a "team" or "household".)
Better career opportunities have made it possible for many women to earn as much or more than men. Nevertheless a lot of these women still haven't let go of the image of having a husband or mate who earns (more) than them.
Imagine if Oprah, Madonna, Mary Barra (CEO of General Motors), Ursula Burns (CEO of Xerox) Virginia Rometty (CEO of IBM) or Indra K. Nooyi (CEO of Pepsi) insisted their husbands/boyfriends had to make more money than them?
They'd probably have a hard time finding a man!
At some point the higher a woman climbs the ladder of success she needs to be open to letting go of the "Cinderella" or "Knight in Shinning Armor" fantasy where the handsome RICH prince sweeps them off their feet.
A lot of women are quick to say: "I don't need a man!" but they also act as though they don't (WANT) a man either.
For many generations men have out earned wives or even supported their household on ONE income without being "ugly" about it. True (equality) would mean if things were reversed it shouldn't make a difference.
Unfortunately the ugly truth is (some) women don't want equality in their households. If they make $100k per year they want their man to earn $150k or more. They resent it or feel used if they are paying most of the bills or their husband was a "stay at home" father. Society even frowns on "stay at home" husbands/dads!