I would guess a combination of 1x and clutch mechs plus advances in cleaner/less sticky lubricants.
BITD it was (for me anyway) mainly a mountain bike problem and usually a middle/granny ring issue.
A filthy triple + clag coated chain + weaker rear mech springs. The mech cage is higher and pointing more backwards towards it’s relaxed position (in the inner ring(s)) and therefore the chain line is closer to level front to back rather than slightly down (as would be found in a 1x).
On a 1x system your rear mech is most stretched / spring under more tension when under the most arduous riding conditions (i.e. uphill in filth) so you get good rearward pull. The size of the rear sprockets also means the lower jockey wheel is lower relative to the chainring so the chain is more likely being pulled down by a long cage mech and away from the chainring not straight back.
Add in materials improvements as well and it’s probably not that surprising.