It's more of a tradition, really. There are plenty of other formal fonts out there to use, but Times New Roman has been the standard for that kind of thing for decades.

A business letter is a formal document used by companies for professional ... When writing a standard business letter, the preferred fonts are either Times New Roman or Arial, especially if ...

Business letters are usually much more formal... What Is the Appropriate Way ... providing a consistent look for the recipient. Fonts are categorized into two areas: serif and sans serif.

Avoid using unusual fonts that distract from the content ... While the tone can be friendly, business documents are written in a formal style. Your writing might be read by individuals other ...