Being listed as one of their 20 most frequently used law firms in America, according to the latest corporate counsel survey, probably boils down to size; the larger the firm, the more likely it is to be cited. This hypothesis makes sense because more law departments have an opportunity to use a firm that has a larger number of lawyers. And, the more clients choose a particular firm, the more the firm can add lawyers. Cause and effect work both ways.
The second reason for dominance of brand-name firms is the “Buy-IBM” effect. In the 60s and 70s, no IT staffer could be assaulted for having bought from the industry leader, IBM. Nowadays, a score of household-name law firms (when you reach Cher status, with a single name, you have made it – Cravath, Skadden, MOFO, S&C) serve as the unimpeachable IBMs for general counsel who fear being second-guessed.
Third, it would not surprise me to find that the most frequently cited firms are also geographically closest to the largest number of major corporations. A Denver-based firm will have a much harder time cracking the list than a New York firm.