Because 20 year old technology has at least a 20 year track record.

The label on the AC Delco Dex VI jug I have says compatible with Dex III so pour away!

Since this is an oil thread, "better overall" is subjective. Later formulations are more environment conscious which can compromise engine protection. I don't know how zinc helps engines but there's less and less of it in new and improved engine oil.

Sixto
98 E320s sedan and wagon