Serialized dramatic or adventure-oriented comics. Popularized by Hal Foster's Tarzan, Alex Raymond's Flash Gordon, and Soap Opera comics such as Nicholas Dallis' Rex Morgan M.D. and Apartment 3G. If you're of a certain age, you'll also remember that once there was a Star Wars daily newspaper strip that lasted several years. The four (maximum) frame a day strip format has proven more suited to self-contained gags than it is to a continuing story. This is one of the reasons that the comic book was invented, to decompress the art layout, give more space to slower paced, dramatic stories that the strip format doesn't afford them. Also, space for comic strips has been shrinking in newspapers resulting in smaller reproductions of the strips themselves. This isn't much of a problem for gag strips which are generally rendered in a simplistic style. However, most adventure and drama-oriented strips are drawn in a realistic style that doesn't hold well to shrinkage. Other than legacy titles like some of the aforementioned (many of which now have more limited syndication than they used to), the serialized adventure/drama comic strip seems to be a relic of the past.
Li'l Abner fell into this pretty hard. For a couple of decades (1940s–1960s), it was the most popular comic strip in America by a mile, with an estimated daily readership of 70 million in the United States alone (back when the country's population was about 180 million). The strip produced omnipresent merchandise and even a few live-action films. Al Capp was called the modern-day Mark Twain. Female characters from the strip, such as Daisy Mae, Sadie Hawkins and Lena the Hyena, were part of the Small Reference Pools. It also spawned an extremely successful spinoff character, the Shmoo, which was a cultural phenomenon in its own right. The main reason Li'l Abner died off in popularity was its complete alienation of the baby boomer generation. Al Capp became increasingly conservative in his later years, and the strip started taking regular potshots at the civil rights movement, hippies, and anti-war protesters. Perhaps even worse, in 1971, Al Capp got caught in multiple near-simultaneous sex scandals that led to many newspapers dropping the strip out of protest. Due to these two factors, the strip's popularity plummeted in the 1970s, until it finally ended in 1977. The result is that today, the strip is a footnote in the history of American pop culture if it's even remembered at all.
Cathy. When the strip appeared in the 1970s, it was considered ahead of its time. The idea of a newspaper comic with an adult, single, female protagonist was rare enough. A working one? Completely unheard of. It was extremely popular in its heyday, with many women readers readily identifying with the title character as she dealt with common problems of modern single life. It even managed to be adapted into animated specials in the 1980s, one of them actually winning an Emmy. Then, time moved on and Cathy... didn't. With the times changing and single working women in media becoming commonplace rather than a novelty, people looked past that factor and realized that it didn't really offer much outside of that. Its jokes about the differences between the sexes aged like chocolate left in the sun: funny and topical back then, really outdated and somewhat sexist now. Its popularity plummeted so fast that when it ended in 2010, the general reaction to the news was surprise at the fact that it was still running. These days, it's only brought up for parodies and mockery (an appearance on Office Space proves this point).