It’s a combination of factors. First of all, you’re absolutely right, the level of artwork that is required by the audience today is greater. We often have an idealized memory of the past, recalling the best stuff and forgetting that it was the best stuff–that not everything produced in that era was at that level. Additionally, today there’s the almost certainty that every job that gets done will remain in print in some fashion indefinitely. So while in the old days an artist might be willing to cut corners and jam through an issue to get it done or to help somebody out, nowadays he knows that the pages in question won’t be forgotten in a month’s time, they’ll be there virtually forever. And finally, and this is a big part of things as well: the pay for a comic artist is better now than it was then, so the average artist needs to produce fewer pages in order to make a sustainable living. Nothing will make you draw two books a month like needing to draw two books a month to keep a roof over your head–but those two books a month aren’t going to be as well-drawn as one book produced under more sane conditions.