Sean & Jim,
Tilly never explained his rationale behind this ranking. I, too, have always been fascinated with wondering why... I think what intrigues me most of all is how many of those Top 9 were still playing competitively when he wrote this in 1939, with none being the answer. Yet if Tilly had written this just a scant 10 years later he most assuredly would have had Byron Nelson in his top 5 and Hogan rounding out in the next 5 somewhere.
It just goes to show, at least in my mind, that as much as time makes it fruitless to compare it is also the supreme equalizer...