Period to the twentieth century, during early days of the nation, in order to unify the language and help people to communicate, the Chinese government started to promote "Putonghua" and put it in Constitution in 1982: "The State promotes the nationwide use of Putonghua [common speech based on Beijing pronunciation--Tr.]." (Constitution, Article 19). Indeed, the promotion of Putonghua unified the language, but also contributed to the disappearance of dialect.
And we think it might have some factors cause and accelerate the disappearance, Education and Entertainment.