One of the longest running gags in Spongebob Squarepants is the feud between the Krusty Krab and the Chum Bucket. Plankton is always trying to steal the Krabby Patty secret formula from Mr. Krabs, and every time he fails, leaving the Chum Bucket unable to compete with its rival across the street. Because of this, Plankton is forced to serve chum at his restaurant (hence the name), and hardly anybody in Bikini Bottom ever chooses to eat there because of how disgusting the food is.
Despite the lack of business, the Chum Bucket is still open after 12 seasons. This is most likely due to the floating timeline, where cartoon characters don't age and the world they live in only evolves when the plot demands it. While there's still no explanation for this, a fan theory offers a logical explanation for why no one likes the Chum Bucket's food.
Reddit user Pasta-hobo posted a theory explaining why the Chum Bucket fails with the residents of Bikini Bottom, and it has ties to real-world biology. The Bikini Bottom population is primarily bottom feeders (lobsters, mollusks, etc.), and bottom feeders are primarily detritivores. This is a form of omnivore that feeds off dead organic material such as dying plants of animal feces. Spongebob creator Stephen Hillenberg, who was also a marine biologist, confirmed that the Krabby Patty contains no meat whatsoever, whereas a Chum Burger is made entirely of meat. Thus, most residents of Bikini Bottom don't like the Chum Bucket's food because it doesn't fit with their natural diet. There are a handful of instances in the show that seem to contradict this theory, but the post offers some interesting explanations for them.
First there's the fact that the original Krabby Patty formula contained some chum. The Season 5 episode "Friend or Foe" reveals that Mr. Krabs and Plankton were once business partners, but during a dispute, the recipe for the secret formula was ripped in half. Plankton's portion only reads "a pinch of chum," so without any other information, he bases his new restaurant over the only part he knows for sure.
However, if Krabby Patties contained chum, why did the residents of Bikini Bottom eat them? This can by explained by the fact that small bits of meat and "chum" can be found within the material that detritivores usually eat, so a small pinch wouldn't affect them very much. There was a point in the episode where a character was poisoned by Plankton's version of the burger, but that was because Plankton simply undercooked the chum.
There are also instances of certain characters eating chum and being perfectly fine afterward. First, there's Squidward, who cooks a gourmet dish titled Chum Fricassee in the Season 8 episode of the same name where he quits the Krusty Krab to go work for the Chum Bucket. This has a biological explanation: Squidward is an octopus, and octopi are carnivorous. Chum contains salmon, which octopi have been known to eat in the wild. It makes sense for Squidward to be okay with eating chum, and because he is known for his snooty personality and love for fine dining, cooking a gourmet dish out of chum is right up his alley. This would also explain why the people who eat Squidward's chum aren't grossed out by it. Plankton is known to be a terrible cook, so Squidward's fancy dish would easily since in comparison.
The theory still doesn't explain why the Chum Bucket hasn't gone out of business after all this time. Those who commented on the theory have theorized that a small but loyal base of carnivores have been keeping the restaurant afloat all this time. Other carnivores, like sharks, have appeared on the show occasionally. After all, Plankton needs to fund his evil plans somehow.
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