Why is District of Columbia not a state?

Can you explain to me why the District of Columbia, often referred to as Washington D.C., is not considered a state within the United States of America? I understand it's the nation's capital, but what are the specific reasons or historical factors that led to its unique status as a federal district rather than a fully-fledged state? It seems intriguing that a place with such political significance is not granted the same rights and privileges as the other 50 states in the country.