Anvilicious: The strip was quite shameless about hammering out Al Capp's various views. This got worse as Al aged and became more aggressively conservative, which led to the strip's sharp decline in popularity.
Overshadowed by Controversy: In 1971, Al Capp came under immense media condemnation due to being caught up in multiple, near-simultaneous sex scandals. As a result, many newspapers dropped the strip out of protest. Combined with how sharply Al Capp had alienated his potential fanbase since the 60s, it was a death sentence.
Seasonal Rot: The strip fell into this pretty hard. For a couple of decades (40s-60s), it was the most popular comic strip in America by a mile, with an estimated daily readership of 70 million in the US alone (back when the country's population was ~180 million.) The main reason it died off in popularity was because of 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, including an infamous feud with John Lennon. 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 70s, until it finally ended in 1977 by Capp himself.