In those days the artists were often not particularly talented, but just happened to fall into the right job. There were some very talented people there as well, but they were often under immense deadline pressures and on very low page rates.
Modern artists are generally people who actively sought out a career in comics and have enough time to draw the pages to a higher standard. They also have better access to reference material (the good kind not the Greg Land kind).
Obviously there were some incredible artists back in those days (Eisner, Toth, Wood, Kurtzman, Kubert, Wrightson, Frazetta) who produced great work, and there are some terrible artists somehow getting work now - but generally if you compare a random page from then with a random page from now the modern page is going to look better.