Another theaory on why High Number series are rare

[Kevvyg1026]

Certainly, in the early 60s, the high series' have more speculative cards (rookie stars, etc) but they do have all-star and MVP cards, which obviously display some superstars. And, by the 70s, some of the interest in the previous decade's superstars (Mays, Aaron, Gibson, Banks, Rose, Yaz) might have been fading (from a consumer interest standpoint) and turning to players like Bench, Munson, Garvey, Reggie Jax, etc. Certainly, as 1970 cards illustrate, the semi high and high series cards, can be populated with both fading stars and up and coming stars.

Moreover, from what I can tell by looking at ads in baseball magazines, many times these series were available in June or July from major card distributors.
So, my opinion is that Topps simply printed smaller quantities of these series, anticipating lower demand from retailers and consumers in mid-to-late summer.