It's clear that it's not about doing "the greatest good for the greatest number." If that were the case, the Forum's top priority would be debating the development of motivational strategies for altruistic action. Any attempt to rationalize altruism must start from the obvious fact that altruism is a phenomenon of individual motivation.
Almost all the Forum's content revolves around how to spend resources one doesn't have. At a time when a genuine humanitarian catastrophe is unfolding due to the cessation or reduction of donations to charitable causes internationally, here they debate longtermist questions, animal welfare, or Artificial Intelligence. Pure Byzantineism.
It's obvious that EA is the private property of its leaders and that, therefore, they can do as they please. Without knowing the personal circumstances of these leaders, we can't know what their intentions are. Perhaps media attention on what are considered "big issues" in certain exclusive circles?
At least thanks in large part to the initiative of someone as prominent as Peter Singer, the idea of "effective altruism" came into being. The rationalization of altruism, the possibility of ideological change beyond religion or politics, are possibilities that have, at least, been articulated.
I think most of what you're after already lives under the EA community building topic.
I'm co-director of EA Netherlands, and the main reason I think this work might be a good use of my time is precisely that it helps develop motivation for unusually effective altruistic action. So any time I'm discussing community building best practices, I'm in effect doing the thing you say should be the priority - 'debating the development of motivational strategies for altruistic action'. There's a fair amount of it about; it's just filed under a heading you may not have looked at yet. And working out what to be motivated about is part of the same project, which is why those object-level debates about animal welfare, AI and so on aren't a distraction from it - they're the substance it's meant to motivate.
I'd also gently push back on the opening premise. 'The greatest good for the greatest number' is the standard shorthand for utilitarianism, and EA isn't the same thing as utilitarianism - so it's not quite right to treat that slogan as the benchmark EA fails to meet. The only commitment effective altruism really requires is that helping others matters. Plenty of effective altruists aren't utilitarians and care intrinsically about things like rights, freedom, or fairness; most people end up giving some weight to several ethical theories at once. There's a good summary of this in the FAQs on effectivealtruism.org under 'Is effective altruism the same as utilitarianism?'.