Such a question cannot be divorced from political, economic, and social context. This may seem obvious, but there are really two sources of change in the textbook market. One is internal: having to do with the dynamics of the market itself, as well as the content and style of the existing texts. If you hear Mankiw or some other orthodox economist talking about why particular texts are successful, they will most likely turn to this flavor of explanation because it tells a story of individualist success and ingenuity in the market -- how textbook writers come up with excellent new ways of presenting economics research in a consumable fashion to first year undergraduate students. People arguing the "internal" view often lack a perspective on why long-term trends and changes in those trends occur.
The second source of change is external: having to do with the social forces alluded to above. (Does a better job of explaining the long swings in a market.) Philosophers and others thinking about the progress of the discipline from the outside are more likely to talk about these sources, but they are just as important (if not more so) than the internal ones. Did the success of Samuelson have anything to do with his commitment to a consumable distillation of the Keynesian mainstream of the time? Most certainly. Did it have something to do with the way he treated the Soviet model and radical economic alternatives? You bet.
So the funny thing is, McConnell always kept in line with social context. For example, in all editions of the text he has included sections at the end of each chapter titled "The Last Word", which included critiques of, say, the mainstream treatment of monopolies by Paul Sweezy. The point was to present economics as having more than one side, to treat that idea carefully, and leave it open to the student to discuss (maybe in class?) the merits of each side.
McConnell and Brue maintained these "Last Words" throughout their textbook, mainly because they were an effective way of presenting an opinionated argument that, in some way, reemphasized the core theme of the chapter. (Samuelson and Mankiw have something like it as well, for a similar reason; it's an effective tool!) But where the earlier editions of "Last Word" had something like a critique of the main theme of each chapter, by the 1980s and certainly by the late 1990s, the editions' "Last Words" were geared more towards an opinionated affirmation of the underlying theme. For example, a chapter on the pure competition model has a "Last Word" on how pure competition maximizes consumer surplus, demonstrating the welfare properties of the pure competition model in a one-page summary of the chapter.
In short, while Samuelson had kept some Keynesian grounding to his text, McConnell and Brue were able to adjust the focus depending on how the majority of economists changed their views.
What does this mean for Mankiw? As I noted earlier, the key invention for Mankiw was the reorganization of welfare economics. This reorganization is a political project in the sense that it "glosses over" key details of the arguments needed for welfare economics to actually make sense. It allows clear applications to the world of public policy. But still, it is, at its core, a right-center book in a world where the mainstream is beginning to shift slowly but surely. As soon as a (mainstream) book comes along that represents economics with more of a political economy focus, more behavioral economics focus, and a better emphasis on the logic (rather than the rhetoric) of core models, Mankiw's position will be in jeopardy.
As a result, I suspect that very soon we will see the downfall of Mankiw's Principles text. But, more on that in another post.