I wonder if personality subtypes play a part in how likely they are to react. I stumbled upon a document about them that also mentions something about social levels. Most villagers mentioned here as not reacting much are in a subtype with low social level. Perhaps this also affects how often (and how exactly) they react to your own reactions, which leads to Crankies overall reacting the least to reactions due to the majority of them being in the low social subtype.
Though I suppose the main personality type could also be a factor, however I’m mostly theorising about all of this. I haven’t reacted much to most of my villagers, let alone the two that fit under the low social category, so I have no testing/ideas of my own through playing. (It’s also just one document, though for me it brings some clarity as I was curious about the different waves some of my villagers do, so...a case of “there are potential truths, but maybe take it with a grain of salt” perhaps. After all, Static is supposedly in the same group but he reacts often according the poster directly above me.)
This is the document I found with the tidbits on villager social levels.