I think they are good at listening what most of their customers want, and at the final deliver it.
I mentioned this, briefly, in the OP when I said, But fundamentally, WoTC/Hasbro did a lot of playtesting, and released the product that a lot of people wanted.
I really think that this is something that drives a lot of what I was thinking; sometimes, there is this whiff of elitism (or more than a whiff) that comes in when I see certain people rubbishing D&D. It's the whole, "People who like it just don't know any better ... if they played Brand X, then they'd come around and see what real TTRPGs are all about."
It never seems to occur to a lot of people that 5e is popular because it's giving a lot of people what they want! That all the playtesting before 5e was released, and the continual UA/Playtesting/Survey cycles we keep going through, is to make sure that we keep getting what we want. It's similar to those who say, "Yeah, those uncultured heathens listening to their hippity-hop music. If they listened to opera, then they would be listening to the real music." That's not to say that opera is bad (or any other subjective preference), but there is often a reason that people like what they like, and it is usually more fruitful to try and understand why people like the things that they like than to assume that they are wrong for liking it.