Here's why:
* Bangla is the official language of Bangladesh. It's the language used in government, education, media, and everyday life.
* Sylheti is a regional dialect of Bangla. While spoken by a significant number of people in the Sylhet region, it's not the dominant language across the country.
While Sylheti has its own distinct features, it's still considered a dialect of Bangla and most Sylheti speakers can understand and communicate with speakers of standard Bangla.