Why do Isaan people speak Laotian?

In my area of Isaan, my village speaks Lao Thai, but two kilometres down the road the village speaks Pheu Thai, in other parts of Isaan they speak a form of Khmer, and my wife tells me in parts of Korat they speak another dialect.

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You're thinking about Sooay spoken in Surin, Sa Kaew, Buriram and Si Saket. For me it is the most musical of all Thai dialects when not hollered by some toothless old crone on a market stall....Our old maid was a native Sooay speaker from Khun Harn in Si Saket but would always deny the Khmer roots in the language apart from it sounding very much like Khmer.