Maybe accelerated plan leakage - loss of assets on the RK platform = lower asset-based fee revenue? Responses might be masked differently, but honestly, how many RKs out there really want to make it as easy as possible to get assets off their platforms?
The one legit reason might be cyber-fraud concerns, where potentially any account could be targeted for a withdrawal, not just those of separated or 59 1/2 in-service eligible participants. But if that is an RK's concern, then maybe they aren't doing enough cybersecurity now, so that may be a red flag.
These are just my anecdotal thoughts from the recesses of my brain, for what they're worth, not any official position or expression of observed occurrences.