Is there a technical reason that new (Apple Silicon) Macs need less RAM?
The 'technical justification' is that the RAM in the Apple Silicon chips is part of the chip itself, it is not a separate entity like all other processors before.
Presumably, they'll perfect how to enlarge this with successive generations.
The broad strokes of how they do this are on the announcement page - https://www.apple.com/mac/m1/
You could probably consider this first generation to be a more than adequate 'proof of concept'.