During some periods Bronze in much of Laos and far northern Thailand was at least partly alloyed with tin from Gejiu in Yunnan to the north.

In terms of logistics: geologically, most tin in northern Thailand / southern Laos is much further west along the Myanmar border or in deep mountains on the Laos/Vietnamese border which (either requiring a long overland journey or an upriver trip toward Ban Chiang / Vientiane) is a lot harder to access than just floating down the Mekong from Yunnan.

In terms of supporting archaeology: Dong Son bronze drums. https://en.wikipedia.org/wiki/File:Dong_Son_bronze_drums.JPG Note the density of findings directly downstream of Gejiu in the Red River valley which flows to Co Loa / modern Hanoi.