The short answer is that the approach taken by J1939 and likewise standards (aimed at legacy motor applications or industrial systems) is unfit for the types of applications targeted by UAVCAN. You may notice that they are being augmented, and at some point will be replaced, by more capable data exchange systems (e.g. SOME/IP, AFDX), because such legacy protocols are unable to meet the data exchange demands of modern systems. This is somewhat covered in this long post here (not in-depth, unfortunately; I may prepare another post focusing on our motivation at some point later): Alternative transport protocols.
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