So TG made a little bit of money because they do not serve wines in BKK C lounges. And the liquor selection is not good or large as suggested above.
Note that the international lounges at the airport are in the dutyfree section, so I doubt that TG has to pay duties on liquors served. So the basic answer is cost-cutting. Similarly food served in BKK C lounges is very very basic. TG lounge in HKG has much better food/drink options.