Why did French become the main common language in Brussels?

Historically, Brussels was a Flemish-speaking city. The shift toward French as the primary language was driven by several demographic and social factors during the 19th and 20th centuries.1

While Brussels remained a Flemish city for centuries, the 19th century saw an influx of newcomers from both Flanders and Wallonia. The linguistic transition was largely influenced by the social status of these groups:1
* Social Stratification: New Flemish arrivals typically belonged to the lower strata of society, such as laborers and domestic servants, whereas Walloon newcomers were predominantly middle-class employees and civil servants.1
* Prestige and Pressure: French held significant social prestige, and as a result of social pressure and the perceived status of the language, a large majority of the population had adopted French by the mid-20th century.1

Although Brussels is officially bilingual today, French has become the lingua franca of the region, spoken fluently by more than nine-tenths of the population. Conversely, the use of Flemish has declined, while English has emerged as the most common second language for work and school.1