You just started studying Indonesian, and one day you go to Malaysia. You notice that a lot of words are familiar to you. You know from sources you read that Indonesian and Malaysian are basically one language. Why is that so?
Since a long time ago, Malay language that was spoken in northeast Sumatera has been used in Indonesian archipelago for hundreds of years. As lingua franca, it is mainly used as the language of trade even by some of the eastern part of now Indonesia (of which the native languages are of Western Papuan Family).
During colonialization, Malay was significantly used as a trading and political language. The Dutch were reluctant to promote the use of their language, and so Malay was popular among the commoners while Dutch is used by the elite, in contrast to other colonialists (the French, Portuguese, and even the British). This was probably done because the Dutch did not want the Indonesians to see themselves as equals to them.
Despite the Youth Congress in 1928 which agreed on the use of Indonesian language as the unification language, the Dutch language was still dominant in a lot of formal aspects. However, in 1938 Indonesian was used in the people’s council (Volksraad) much to the Dutch’s chagrin. By the time the Dutch realized that the use of Indonesian language is a threat to their interest, it was too late.
Now, the question here is why didn’t Indonesian founding fathers choose Javanese? Javanese as an option really made sense. First, Javanese kingdoms were powerful and dominant. Second, It was (and still is) the language with the most number of first speakers in the whole archipelago. Third, a lot of the founding fathers are Javanese, including the first president (whose father was a Javanese aristocrat).
The reason is very simple. Javanese is not a simple language. It’s very impractical. When you learn Javanese you have to learn three different styles; Krama inggil, krama madya, and ngoko. If you think it’s easy, think again! Those styles are basically different languages not because of verb inflections (changes in the verbs to signify different functions); they have different sets of vocabulary. For every one thing (or idea), you have to remember three different words! Not only that, you also have to know where and when you should use those languages.
Have I mentioned that even though you are using krama inggil (the most polite and formal style), you have to be able to choose when to lower yourself or honor someone else. There are different words for that also. In fact, what’s confusing is that you can answer using a krama inggil verb to show your superiority by using honorific verbs. Confusing? I know.
Now you know why Malay is more preferred and practical and why it was chosen as Indonesian.