Funnily enough, IBM had already published this. CUA explicitly says tab and backtab move between fields.
so they spent seven layers of management escalating against their own standard: https://archive.org/details/ibmsj2703E/page/n13/mode/2up
so they spent seven layers of management escalating against their own standard: https://archive.org/details/ibmsj2703E/page/n13/mode/2up
Another factor could be still-reverberating echoes of the likely political battles around something as broad and far-reaching as CUA. I can only imagine the quiet boardroom battles won and lost fighting over CUA between different factions across all of IBM's kingdoms, divisions and principalities.