I would think those characteristics would allow some kind of monetization via ads etc. I assume google looked at the numbers and saw that is not the case, and not simply at user growth. Cash cow vs growth vehicle kind of thing.
If i had to guess, it seems likely that RSS reader users are more likely to have ad-blocking enabled and less likely to click on ads if they see them. perhaps google wants to skew towards the more easily misguided.
Ultimately, if people log in less, and contribute less to the Knowledge Graph (crowd sourcing production of meta-data had to be worth something, after all) then its a net loss.
Google's experiments may cause them to believe this not to be the case, but the trouble with ruthless, consistent optimisation of individual features is that it tends to land you in a local minimum, which may not be the best end game for them.