Soft errors from SEUs/SETs in early 8-bit microprocessors?
Was it because the transistors in those early microprocessors were microscopic instead of nanoscale?
Mostly yes. Larger geometries and higher voltages are less susceptible. Early processors ran on 5V, now processor cores are in the 1V range.
You also must consider the consequence of an SEU. Most people were not doing serious work with the processors that you mentioned, they were playing games.
For many applications, the probability of a S/W bug causing lost data is much higher than the probability of an SEU causing lost or corrupted data. As long as this is the case, worrying about SEUs isn't the highest priority.