They basically had two choices once they did introduce synchronization: Keep attestations around, but specifically mark synchronized credentials as "not strongly device-bound" (and risk existing relying parties not looking for that flag and drawing incorrect conclusions from receiving such an attestation statement), or get rid of it entirely.
I suspect that they opted for the latter mostly because it would require a lot of work with the FIDO and WebAuthN working groups to introduce that mechanism, not out of a selfless desire to avoid a future "big tech lock-in" (where everybody allows exactly Apple and Google passkeys, but nothing else), but I could definitely see the latter consideration also playing a role.