Well, it's incredibly hard to scale fraud only through person-to-person communications. It's not impossible, but think about the scale that online communication would allow.
This has nothing to do with scaling. How do you picture that, some employee would broadcast sensitive information to a WhatsApp group, in order to reach as many others as possible at once?
The offense here is that no effort was done to keep records of the communication. It would have been ok to use WhatsApp if they somehow would have archived all communications.
Records of communications have to be kept so that auditors can verify that no inside trading secrets were communicated to others, for instance.
Now I understand why some of the banks wanted people back in the office as soon as possible. Trying to support their commercial real estate bags is likely another.
The offense here is that no effort was done to keep records of the communication. It would have been ok to use WhatsApp if they somehow would have archived all communications. Records of communications have to be kept so that auditors can verify that no inside trading secrets were communicated to others, for instance.