Need help with baking texture in Blender :)

Hey! It's because your mesh is double sided. What is more it should not be double sided, you should close holes at the bottom and top, not make it double sided. Bake with higher margin (10+) and with this fabric only thing you can bake now with full render is white color, you should bake in ambient occlusion or if using full render to bake change settings so it gives at least a little shading.
Retransfer uv_1 using Nearest Face Interpolated, it'll give you way better results than what you have now