Because they wanted to focus on the switch with the short time they had. The switch is the big thing they need to show. Nintendo needs to convince everyone the switch will be worth it.
My question is WHY is it not coming to the Switch in HD?!?! This would have sold SO WELL on the Switch. And this is the type of Metroid game I want to play again. This could have at least held Switch players over until MP4 releases god knows when.