Most probably the earlier engines that used plain small ends were slower running, less powerful & ran on thicker mixes, so manufacturers were able to use a cheaper solution while getting a reasonable life. Replacement when required was cheap & easy too.
As power outputs & engine speeds increased, along with a drive to reduce mixture ratios to reduce pollution, plain bushes couldn't cope & the move was made to using roller bearings to get better reliability along with reasonable lfe.
Downside is that repair became more expensive, as the conrod has has to be changed if the hardened & ground eye wears enough that normal operating clearances can't be obtained with a new roller bearing & gudgeon pin.
A lot more oil sloshing around in 4 strokes - many have a jet that sprays oil under the piston crown to both cool the crown & increase the amount of oil in the gudgeon pin area.