For insular SEA it was because of the spice trade (see “The sea as the focal point,” The Manila Times, Oct. 29, 2020), but for continental SEA, it was due to the immense China trade.
Providing vital services to foreign merchants made Funan a major SEA power until its collapse around the 6th century CE.
In the years of prosperity and power, Funan maintained a tributary relationship with China, where its ambassadors had to kowtow before the emperor.
However, China giveth and China taketh away.
Contemporary China insists so.