Take a close look at this table, which tabulates sales data for Samuelson's and McConnell's texts between 1948 and 1984.
|From Kenneth Elzinga, "The Eleven Principles of Economics," Southern Economic Journal. Vol. 58, No. 4, 1992, pg. 874.|
While you could argue that there were some isolated years in the mid- to late-1960s when McConnell outsold Samuelson, upon closer look at the edition release years you can see that it's not until McConnell's 4th edition -- in press from 1969-1971 -- that it outsold Samuelson's competing new text, the 8th edition -- in press from 1970-1972. The previous editions of the two texts were certainly competitors, but Samuelson won by a hair in the previous edition wars: 389,000 for his 7th edition vs. McConnell's 378,000.
Also, notice the dwindling sales of the 11th edition of Samuelson (and the long revision date! for a while, both texts were on a 3-year cycle). For the 12th edition, released in 1985, McGraw-Hill brought Nordhaus on board as a cowriter of the text in order to try to reorient the ship. One of the big problems with Samuelson's book was its explicit Keynesian focus -- no doubt a product of the seminal intellectual and political influence of its author, but something which in the end would harm a textbook writer, since one needs to be able to adapt in some way to the shifting political winds (particularly if you're writing in the mid-70s, early 80s!).