I haven't looked at the research recently, but my understanding is that while more frequent service is strongly correlated with higher average passenger counts,

I haven't looked at the research recently, but my understanding is that while more frequent service is strongly correlated with higher average passenger counts, the causation seems to be that higher average passenger counts justify increased bus service, and not that increased bus service generally results in higher passenger counts. Better route coverage likewise. Reduced prices are complicated. That does increase usage, until the prices drop so far that you attract homeless people who your would-be passengers are scared of. It turns out that making mass transit passengers feel safe is a very important consideration.