Answer: Prior to the emergence of Islam, communities in Southeast Asia already had an established trading relationships with traders from the Arabia Peninsula.
Explanation: Islam expanded into Southeast Asia, first along the trade routes used by Muslim merchants, then spread by Sufi orders, and finally by conquering and expanding the territories and converting their rulers into Islam.
As for the trade routes, according to historians, Arab traders dominated the commercial routes in Southeast Asia at the beginning of the ninth century. The main trade route was between West Asia and the Far East. The first Muslim communities appeared in the seventh, eighth and ninth centuries in this area, but Islam did not expand here significantly until the twelfth century.
The communities in South East Asia have had good relations with Arab traders and before the emergence and establishment of Islam. Thus, these relationships have been maintained over the centuries, contributing to, after the founding of Islam, its easier spread in this area due to the existing connections with the Arabs. Also, Muslim traders controlled a significant part of the Indian Ocean trade route, and had direct access to commercial ports without the need for intermediaries, and facilitated and speed up trade.
After the flourish of trade ties between Western Asia and this area, many Muslim merchants settled parts of Southeast Asia, for example, in Sumatra, the Malay Archipelago, and marriages between Muslim traders and locals in this part of Asia.