I see Reddit, Facebook, Topix, Google+, HN, all providing "watering holes" or a place for a community to form, and these days there is always an algorithm connected to a mechanism for the community to use to signal its approval and disapproval. In "broad" community sites like Facebook and Reddit there is an explicit 'hands off' policy on how those communities operate (until there isn't). That supports a wide number of communities and optimizes for high user counts. In "narrow" community sites like HN and various forum sites, there is an explicit 'hands on' policy which drives the community to a particular place. That optimizes for content/discussion that is highly aligned with the owner's editorial goals even at the cost of ejecting users who don't align with those goals.

The way I think about it, for a multi-community site to actively "influence" its users, the site operator would have to apply some mechanism for shaming or promoting specific communities within it. Further it would associate users with that level of compliance which would signal to them if they were 'betting better' or 'being worse'. The signalling is needed to help the users understand which way they are "supposed" to go, without that signalling it is much harder.

So multi-community sites don't (and won't) do that because it results in users leaving and taking their eyeballs with them. And ultimately the point of such sites is to collect as many people there as possible with as wide a demographic as possible, so that people can advertise goods and services to them.