Good question.

The way I have seen it: the Queen's children pretty much picked up a significant workload once they finished with careers. The careers ended for a variety of reasons, but once done, they focussed on raising families and royal life events. They did so as the children of the reigning monarch.
William and Harry seem to have followed a similar trajectory: careers and then a focus on raising a family. However, what sets them apart is the fact that they are not the direct children of the reigning monarch, but of the heir.
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I think the bottom line is both William and Harry (at least up to now, and that may change as they bask in the warmth of public acceptance, affection and interest) have not been willing to work much for the monarchy. They do what is the bare minimum. William's failure to be more engaged has always been ascribed to his career, but now it's to do with raising a family (which really shouldn't impact, as we all know everyone does both: work and family). It's Harry who is doing very little since the army but no one seems to mind.
I think we will start to see a new Harry with his marriage. He seems inspired and will likely start to engage the life he has up-to-now deflected away from.

So my first answer is: Harry has taken a bit of time to 'mature' into his destiny role. With marriage I think he will answer that question with his activities with his new wife. Just a hunch.