From Jean Ure:
I'm afraid I'm going to be exceptionally boring and say that I agree with you.I am loth to criticise Austen, but in fact the more I think about it the more I find her treatment of Henry to be artistically somewhat lazy, not to say facile. It's pretty standard behaviour, indeed even a literary cliché, for a charming ne'er do well, having made an attempt at constancy, to revert to type by racing off with the nearest available female and ruining her. Henry was at his most interesting when Austen allowed him genuine feelings for Fanny; at his least interesting when he predictably fell from grace. Now, if he had remained true, but she had still married dear old stuffy Edmund, that really would have given readers food for thought. And might have made an even better book. But Austen's strength lay in her wit and her shrewd observations, and to have allowed Henry to develop into a more fully rounded character, capable of deep and lasting affection, might possibly have unbalanced the book by giving him pride of place as the most interesting character, and the one with most potential for a life beyond the pages of Mansfield Park.
My guess is that she was mainly interested in Fanny and Edmund and didn't lavish as much care and attention on Henry as she might have done. I have a sneaking feeling that it simply didn't occur to J.A. to do anything else with Henry. It probably would have occurred to a bigger writer - George Eliot, for example. But Eliot had her faults, of which a sprawling lack of discipline was one. Jane was very near perfection!