I was reading Middlemarch earlier this evening and came across the word "religiousness", which struck me as odd because I would've expected "religiosity" and presumed that adding "-ness" to the end of an adjective to make a noun would've become more popular in more recent times in the same way that the past tense is getting gradually standardised to "-ed".
However, I checked Google ngram,
and fascinatingly discovered this. Middlemarch was published in 1871 (same year Alice bands were invented. Fun fact that I know for some reason) so it makes sense that she used "religiousness".
I haven't done any research (😔) and am purely hypothesising here, but I'm presuming that prior to the 19th century, atheists (or nullifidians I suppose) were so few and far between in the English-speaking world that a term for how religious one was wasn't necessary, so piety and zeal fulfilled its roles.
As for why religiousness came before religiosity? Maybe "-ness" as a default dates back that far, but after it took off, some people began to 'correct' it to a word that fit better, and when that word became popular, it quickly replaced the clunkier version.
No idea though; I'll go and do some research and report back with any interesting findings