Because the old Mercy took a lot of thought and was thrilling, and new Mercy feels so boring. She just does.
For instance, before Mercy had valk part of the game when you played her was paying attention to the tempo of the team fight and finding ways to stay alive and heal as much of your team as possible, and when things got hairy you’d have to be sneaky or fast to get out of dodge in the hopes that you could come in and use res when the opportunity arose. These are things you’re doing, active things, active choices that make you think and give your brain excitement. Also, people were super on the hunt for you creating an extra level of challenge the higher ranked you got - the better you were the more likely a Tracer was gunning for you and only you. It was THRILLING.
Now when I play Mercy the most I have to pay attention to when it comes to the tempo of the fight is just whether or not Valk might be useful right now, which it’s like… IDK, is a lot of my team hurt? Sure, it’s useful. Am I going to die right now? Sure, it’s useful. These aren’t very engaging questions to be asked, it’s not challenging, it doesn’t really push me as a player. Yes, I am getting to put input into the game, but the general scope of it is just so much smaller now.
Now that you can heal even less than before you’ll be simply healing the majority of the game, you’re doing the same thing all game, and your ult will be used to push your team somewhere, and that’s it. It’s about as engaging as mayonnaise to me.