This is more about simplicity, maintainability, and possiblity for new contributors to easily jump in and fix things. Our current parser is not fun for new contributors to learn. Also, I think parser combinators obsficate a lot of the tracking required for robust error messages.
Personally, I find parser combinators nice for small things but painful for large and robust things.
When I used parser combinations in rust years ago the compile times were really long. In also think it’s a strange reason to move away from rust as a language.
Almost all parser combinators are recursive descent with backtracking, they just add higher-order plumbing.
I have a feeling that whatever issue they've encountered with the combinator approach could have been worked around by handwriting only a few pieces.