Bahasa Melayu unify people in Nusantara archipelago. How come Bahasa Indonesia has "Exclusivity"?
The language franca in the strait of Malacca (both the Malay peninsula and coastal Sumatera), coastal Borneo, and the islands in between became Bahasa Melayu (Malay) in the 15th century.
As the islands of the archipelago (now Indonesia) gradually came under the influence of the Netherlands in the 17th century, Bahasa Melayu became the most important medium of communication between government and people for European rulers.
Unlike many other colonies, the local population in Indonesia was not forced to use the language of the European overlords.
Only a small elite of indigenous Indonesians learnt Dutch, therefore Bahasa Melayu (Malay), while being a minority language in the Indies, was critical to the colony's successful administration. When the Japanese conquered the Netherlands East Indies in 1942, one of the first things they did was make the Dutch language illegal.
Because few Indonesians spoke Japanese, Bahasa Melayu had to be employed much more widely and actively in administration than it had been under the Dutch.
With such a long history of use in modern administration, Indonesian was an obvious choice to be the Republic's official language and government language. All government business is now performed entirely in Indonesian, including legislation, administration, justice, defense, education, national development, and so on.
Indonesia's present prestige stems in large part from its participation in the country's nationalist movement. However, in the early twentieth century, Malay was not a clear or universal choice as the language of indigenous cultural and political rebirth in the Dutch East Indies.
Initially, nationalism was expressed as much in Dutch or the languages of Indonesia's indigenous cultures as it was in Bahasa Melayu.
The renowned Young People's Vow (Sumpah Pemuda), created at the Congress of Young People in 1928, was the first time the name "Bahasa Indonesia" was publicly accepted, and the language was recognized the preeminent language of Indonesia as well as the language of national unity.
In today's Brunei Darussalam and Malaysia, Bahasa Melayu is the official language. It is also Singapore's official language. Bahasa Melayu is also spoken by the indigenous people of southern Thailand, Myanmar, and Cambodia.
Filipinos speak Sabah-Malay (a pidginized indigenous trade language of Sabah) as a commercial language in the Philippines' far south.
Bahasa Indonesia is spoken by around 198.5 million people, according to the most recent data from Ethnologue. 19.1 million people speak Bahasa Melayu.
Fun fact: Malay or Bahasa Malaysia is not fully "lost" in Indonesia. Malay is on par with other regional languages such as Balinese, Buginese, Sundanese, Bataknese, and Javanese in this country. The term Malay is also associated with the language and the country in Indonesia.
Source: Bilingua.io, Daytranslations.com, Masteringbahasa.com