Question

Why did major league baseball owners support segregation by 1890?

Answer 1

The answer is they feared that white audiences would not pay to watch African American player

Answer 2

By 1890 organized baseball had been racially segregated. Although many members of the baseball community were campaigning for all races to be integrated together in the sport, many other members were not in favor of doing such.  

Many major league baseball owners supported segregation because their teams used to rent the stadiums to the "Negro League" teams when their teams were on the road or did not have a game. If the sport would be integrated, the major team owners were afraid that they would lose that income from renting to the "Negro League" as that league would probably not exist anymore.  Some of them also believed that their team's audience, which was white in majority, would be reluctant to pay for a ticket to watch black players on their team.