Everyone is always quick to place blame on Disney. This decision was made under the direction of Kathleen Kennedy, the new head hand picked by Lucas. Disney doesn't micromanage their companies.

The question then becomes why was this decision made? Most likely it was suggested by the senior franchise handlers after their experience with the Clone Wars.

They're married to the idea of a unified universe and knew that the new people coming in wouldn't want to be saddled with all of the post-ROTJ continuity. And there was no way they'd be able to retcon something as big as changing the big three's children.

So this way they can start fresh and oversee everything to make sure it all agrees and they can work in the best elements of the old continuity. Kennedy new to the company decided to trust their judgement and went along with it.