I have posted repeatedly why I think that this is a ... poor analysis, and it substitutes ipse dixit reasoning (D&D is the biggest because D&D is the biggest, therefore D&D is the biggest) for any kind of useful information.
What does that mean? We know that D&D was first ... but it wasn't foreordained to be #1. It had a lot of competition early. It had a lot of struggles in terms of the company. It had several fallow periods. It had various noteworthy competitors. And the product has varied over time (TSR-era D&D is not the same as 3e is not the same as 4e is not the same as 5e).
I prefer to think of "why" something is the way it is. For example, I can talk about a lot of different industries, and I can explain why something might continue to succeed even in an undifferentiated market (usually due to advertising, brand loyalty, selective adoption of changes, etc.) and why it doesn't (disruption due to competitors, price competition, use of inferior materials when making it, etc.).
To me, the question of "why" makes the question interesting, because it can lead to actual conversations about design decisions- not just optimal design decisions, but also design constraints. This is what I often see lacking in a lot of conversations about TTRPGs- people ignore the actual market for a game, assuming X market (people like them, people they play with, an idealized consumer) as opposed to what the mass market might actually want.
Put another way- if I was a car designer, I wouldn't design pickup trucks. Yet I look around the road, and what do people keep buying? Pickup trucks.
EDIT- and this is why I think Daggerheart (and Shadowdark, and other games) always present interesting examples. Actual sales? They are revealed preferences. If Daggerheart is wildly successful, that will tell us a lot about what the mass market wants now, right?
