Well I think he didn’t have quite the charisma and fun, trash talking style of an Ali or Tyson (or even some of the foreigners Americans love like Fury or in UFC, McGregor). I think a lot of Americans like the trash talking that can be sharp and biting but doesn’t take itself too seriously. Ali had an incredible way of words, and he could be sharp and insulting, but it had a playful style. Mike Tyson could say some dumb, odd sort of things, but also could come across as a playground bully in a rather amusing sort of way. I don’t think we really have an American fighter since Mike Tyson with that sort of charisma and charm. For awhile I thought Wilder might become that, but he seems to take everything too seriously and acts like the world is out to get him. And getting back to your original question — Ward didn’t really trash talk much, he had a very quiet, almost humorless demeanor, and I think that was a bit of a turn off for all but the most devoted fans.
In addition, as for fighting styles, Americans tend to like boxers who give them their money’s worth in the ring. They want to see incredible punch dodges, and amazing knockouts. In general, all but the most nerdy type boxing fans tend to get a bit bored with the sort of mauling, holding and clinching that Ward used to great effect. He was no GGG (whom you mentioned) who kind of always did the game plan of going toe to toe throwing power punches at his opponents, usually tearing through them like a lawnmower, at least until he reached the top level where he had a bit rougher of a time (see: Jacobs, Canelo).
I see in your original post that you labeled one of his attributes as “slick”, but I don’t really see that. He was no Pernell Whitaker. He never really struck me as especially athletic, and mostly mauled and rucked his way through fights (very successfully I might add). And as another poster pointed out, he didn’t really have the sort of boxing Adonis physique that tends to draw admiration from casuals. He kind of had the body of a local pastor or financial advisor who happened to take his health seriously and do some swimming and calisthenics on weekends. But no Adonis frame like Mike Tyson.
As for not making a lot of money fighting, that probably is best answered by pointing out how he didn’t fight often like Canelo, for example, or into his forties like Hopkins, or even just to the cusp of his forties like Mayweather. He retired quite young, and he didn’t fight too often. He didn’t really have the ideal career in my opinion, but he did what he wanted to do and was very successful to the extend that he did fight. No one was able to defeat him, and he fought his share of tough opponents. I think that counts for a lot, and history will probably be kinder to him as time passes.