I'm hoping @WinningerR or @SlyFlourish can respond to this, but anyone is welcome ofc.
I'll try.
First, be careful about using the final total raised by a Kickstarter to judge its success; you also have to look at the rewards they're offering. Some Kickstarters juice up their numbers by throwing in custom minis, tchotchkes, additional books, and the like. The number of multi-million dollar Kickstarters that return thin profits (or
lose money) might surprise you. (This isn't necessarily a comment on any of the specific Kickstarters you mentioned, just a general caution).
That said, as best as I can determine, the four attributes of 3P D&D products that maximize their commercial appeal are:
Mechanical Content: Players are always hungry for new Subclasses and systems. WotC has good reasons for producing a relatively limited amount of new mechanical content each year, leaving plenty of unmet demand in the market.
High Concept: How quickly can potential buyers understand exactly what you're selling? Ideally, just a single sentence and one or two images tells buyers almost everything they need to know. We like to laugh at Hollywood-style pitches, but they work: Spelljammer is "D&D in space." Curse of Strahd is "D&D vs Dracula," etc.
Broad Usefulness: Every DM can use more monsters and spells. How easily can your content be incorporated into a typical campaign? A non-trivial number of customers who purchase setting books, for instance, have no intention to play in the setting; they're looking for inspiration and mechanical components they can graft into their own campaigns.
Professional Sheen: A lot of 3P content looks amateurish. Quality graphic design doesn't have to be expensive.
I'll again note that, IMO, if you're creating something that doesn't use WotC's IP, you're better off using the OGL or CC than publishing through the DM's Guild. Theoretically, something like the DM's Guild should be visited so often, by so many D&D players, that its expanded audience more than makes up for the extra royalties, but I don't believe that's true at present. Looking ahead, I won't be surprised to see WotC merge D&DB with DM'sG to change that.