Here's why:
* Formal writing requires precision and clarity: Colloquialisms are often informal and ambiguous, potentially leading to misinterpretations.
* Colloquialisms can be outdated: They change rapidly, and what's considered acceptable today might sound outdated in a few years.
* Colloquialisms can be regional: They vary from place to place, making your writing less accessible to a wider audience.
* Formal writing aims for a neutral and objective tone: Colloquialisms tend to be more informal and subjective.
However, there are some situations where colloquialisms can be used in formal writing:
* To create a specific tone or effect: For example, a writer might use colloquialisms in a historical fiction novel to sound authentic to the time period.
* When quoting someone's exact words: If someone uses colloquialisms in a speech or interview, it's appropriate to quote them directly.
* When writing for a specific audience: If you know your audience is familiar with certain colloquialisms, using them might be acceptable.
Ultimately, whether or not to use colloquialisms in formal writing depends on the specific context and your audience.
Examples:
Formal: "The professor delivered a stimulating lecture on the history of the Roman Empire."
Informal: "The prof gave us a really cool lecture about the Romans."
Formal: "The company's financial performance has been declining."
Informal: "The company is tanking."
It's always a good idea to err on the side of caution and avoid using colloquialisms unless you're absolutely sure they are appropriate.