Not the whole reason (fatigue over the Renaissance formula didn't help), but there was a real push in the 2000s of Disney trying to target the boy demographic, because the Disney name and a lot of their franchises skewed more towards girls at the time. It's the reason why Disney XD was even created (love you Tele 😘). There were some hits (Pirates), some modest successes (Tron Legacy) and some failures (Prince of Persia), but nothing really stuck in the same way Disney Princesses did.
Then Disney bought Marvel and Lucasfilm, because it's a lot easier to build a boy audience by buying stuff they already like. Honestly the right call for Disney in the end.