If you compare the modern comic books and graphic novels to the original artwork, the difference might shock you at first. The manipulation of color, the facial expressions, and the implied movement of the characters are beyond different. How and why has this change occurred?
Different Art Styles
One of the simpler answers lies in the fact that each artist, regardless of the genre or medium they use, has a different style. Some prefer the simple and basic contours, while others dive into details. Some stick to the boxes, while others let their characters leap from the pages to pull the reader in. While every artist is, in a way, the product of their time, each of them wants the viewer to focus on something else.
Resources
The artists of the olden days were often pressed for time. It wasn’t uncommon for them to have to come up with illustrations for the latest storyline that the company in question needed by yesterday. Combine this with the fact that there were no computers and image editing programs that grant the artists of today all kinds of options to exploit to make their artwork more impressive and you have one of the reasons why the art changed.
Furthermore, the illustrations before email and file-sharing were given either to the post office or the courier in order to get to the printing presses on time. Another factor is that artists often had to use tricks to make the comics more frugal in tough economic times. This resulted in some comics being black and white, with the shadows and colors done not by different shades, but by dots. These days, artists take all the time they need with seemingly inconsumable resources. One more thing: it is comparably easier to make the art and slap a text box today than it was 70 years ago.
Minimalism
Comic books evolved from one-shots and funnies. Somehow, the name stuck and, in spite of the fact that there are some really graphic or gritty issues out there. The funnies demanded a simple, minimalistic art style that was quick and easy to produce, especially for the daily publications. If you are a fan of webcomics that strive to follow the same, or similar, formula, like PvP or CAD, you know that this is no small feat. It is impossible to come up with illustrations that could parry Batman: Arkham Asylum if you have to publish new content every day or once a week. Not that you would need to or should if your goal is to make people laugh.
Message
Going back to artists wanting their readers to focus on something in particular, artists, especially the artists of today, want to convey a certain impression or a specific message in their art. In the comics of the Platinum and Golden Ages, the goal was to sell comics quickly and efficiently. Artists, as amazing as they were, were constrained by time and other resources. The artists and the art evolved to present emotions, strife, dilemmas, and to symbolize all of that through a few lines and shades of color.