Sounds like both the Cubs FO and scouts are quite a bit more confident in Busch's hitting abilities to transfer better at the MLB level compared to Mervis. From Sahadev Sharma:
https://www.bleachernation.com/cubs/2024/01/25/the-cubs-clearly-intend-for-michael-busch-to-be-the-first-baseman-to-open-the-season/
This would explain the comments from Hoyer/Hawkins since the Busch trade and their confidence of him over Mervis. They've said he's going to be on the OD roster, and envision him as a player in the lineup over his 6 years of control from this Hawkins interview I listen to:
https://www.bleachernation.com/cubs/2024/02/05/carter-hawkins-speaks-working-the-phones-being-opportunistic-rotation-buschs-role-free-agents-more/
They also like Busch's glove at 1B from these comments from the first article above:
Looks like Mervis is slated as either a backup depth for Busch, a potential DH option, future trade bait, or Mervis end up hitting well and playing 1B better than Busch, moving Busch to DH. Tons of options available, especially if Bellinger is signed and PCA takes over CF in the future.