It's my own personal best guess that both the "forgiving" of loans a few years back, and now this-- the apparent refusal by the WTS to continue to hold funds on behalf of congregations-- were both as a result of legal advice. In both cases the WTS' activities could have been interpreted by some banking regulators as dangerously close to acting as an (unregistered/unregulated) bank.
The last thing the WTS would want to do is put itself on the hook as a registered bank since it would then have to disclose lots of info to both the banking regulators and to the bank's customers (those congregation for whom they are holding funds, a.k.a., "deposits").