Answer by taking a quote from the following article:

The Sami people (also known as the Sami or the Saami) are a Finno-Ugric people inhabiting Sapmi, which today encompasses large parts of Norway and Sweden, northern parts of Finland, and the Murmansk Oblast of Russia. The Sami have historically been known in English as the Lapps or the Laplanders, but these terms can be perceived as derogatory. Sami ancestral lands are not well-defined. Their traditional languages are the Sami languages and are classified as a branch of the Uralic language family.

For long periods of time, the Sami lifestyle thrived because of its adaptation to the Arctic environment. Indeed, throughout the 18th century, as Norwegians of Northern Norway suffered from low fish prices and consequent depopulation, the Sami cultural element was strengthened, since the Sami were mostly independent of supplies from Southern Norway.  However, during the 19th century, Norwegian authorities pressured the Sami to make Norwegian language and culture universal. Strong economic development of the north also ensued, giving Norwegian culture and language higher status. On the Swedish and Finnish sides, the authorities were less militant, although the Sami language was forbidden in schools and strong economic development in the north led to weakened cultural and economic status for the Sami. From 1913 to 1920, the Swedish race-segregation political movement created a race-based biological institute that collected research material from living people and graves, and sterilized Sami women. Throughout history, Swedish settlers were encouraged to move to the northern regions through incentives such as land and water rights, tax allowances, and military exemptions.  The strongest pressure took place from around 1900 to 1940, when Norway invested considerable money and effort to wipe out Sami culture. Anyone who wanted to buy or lease state lands for agriculture in Finnmark had to prove knowledge of the Norwegian language and had to register with a Norwegian name. This caused the dislocation of Sami people in the 1920s, which increased the gap between local Sami groups (something still present today) that sometimes has the character of an internal Sami ethnic conflict. In 1913, the Norwegian parliament passed a bill on "native act land" to allocate the best and most useful lands to Norwegian settlers. Another factor was the scorched earth policy conducted by the German army, resulting in heavy war destruction in northern Finland and northern Norway in 1944-45, destroying all existing houses, or kota, and visible traces of Sami culture. After World War II the pressure was relaxed though the legacy was evident into recent times, such as the 1970s law limiting the size of any house Sami people were allowed to build.  The controversy over the construction of the hydro-electric power station in Alta in 1979 brought Sami rights onto the political agenda. In August 1986, the national anthem ("Sami soga lavlla") and flag (Sami flag) of the Sami people were created. In 1989, the first Sami parliament in Norway was elected. In 2005, the Finnmark Act was passed in the Norwegian parliament giving the Sami parliament and the Finnmark Provincial council a joint responsibility of administering the land areas previously considered state property. These areas (96% of the provincial area), which have always been used primarily by the Sami, now belong officially to the people of the province, whether Sami or Norwegian, and not to the Norwegian state.

Why was the language forbidden?