Do you have any particular countries &/or time-frame in mind? I'm thinking of counter examples e.g. that the entire Americas were populated via Siberia, the Mongols conquered a vast territory, and China engulfed Mongolia (although they later lost Outer Mongolia) & Tibet.

Keep in mind that a colony is a very specific term and idea. The Okinawan Kingdom, at the height of its independence (14th to 16th centuries), had permanent settlements in a few places in the region, including in China--and both China and Korea maintained permanent enclave settlements in Okinawa. Most of their time was spent engaged in maritime trade, which was much more profitable for them than that weird mixture of conquest and cohabitation that is colonization.

China, during the same time period, was similarly more interested in maintaining regional dominance through heavily proscribed ritual and trade relationships via suzerainty.

At the same time, Asian countries weren't idle. There was plenty of conquest, war, and pillage going on, but it was of the more traditional sort rather than colonization.

The idea of "colonies," insofar as I'm aware, seems to be a particularly Western European notion. Perhaps a better question would be: Why did colonization arise from there during the Age of Exploration?

(I have some thoughts, but they are outside of the proper realm of my area of expertise, and perhaps the realm of history itself.)