Probably mostly just due to laziness. As Tony says, Geoff's books are very thorough, well researched, and well thought out, so it takes real effort to go beyond what he's already done so well. Paul Daley's books are similar in this respect.
That, plus there is a vocal minority on here that enjoys belittling anyone who reads about gca, labeling them as incapable of independent thought if they agree with something they read, and saying they should play more and read less.
Anyone who hasn't read at least a few of Geoff's book is missing out on something special. I miss him not posting much on here anymore.