Why would that group of people have an easier time maintaining a marriage than "average" people?
Top execs are often married to their jobs with less time for family. Actors are exposed often to very liberal and beautiful people (affairs are common). And people on the lower economic rungs are more incentivized to stay together to spend less money.
The research actually shows that the upper end of the income distribution is more likely to 1) marry and 2) remain married. The lower and middle classes have been most disrupted by social change in the last 50 years.
Because they are much more desirable than average people are.