Quechua is known by one name although the various dialects differ a lot, because there once existed this one standard Quechua, a variety that resembled the Quechua language of the Huarochiri text. The Incas adopted this language as a state language because is was already in use in many parts of their empire. Quechua was the language of Pachakamaq, Chincha and spoken as a trade language in Ecuador.

During the colonial peruvian time, a new standard variety arose: the so-called ‘language of the Incas’ or Cusco variety ( Cuzco Simi ). This variety was spoken by Incas, nobility and hispanic colonials, was a different Quechua than the standard Quechua of Tawantinsuyu. The language was spoken in Inkasuyu, a region that lies in southern Peru and Andean Bolivia.

The Inca Quechua was spoken by Tupac Amaru II and after his defeat it became attacked by the Spanish administration, part of a policy to abolise Inca pride and heritage.

Quechua remained the main language of the peruvian andes for a long time, spoken throughout ethnic groups, also by white peruvians in the andes as a mother tongue. A negative attitude towards Quechua began when Andean people went immigrating to the capital city of Lima. Today Quechua is considered to be part of the original Peruvian identity.

Academic knowledge about Quechua and other Andean languages:
http://www.brunel.ac.uk/4042/entertext2.2/itier.pdf

http://peruanista.blogspot.com/2008/07/la-nacionalidad-quechua-en-el-mundo.html

Saludos.