Here's why:

* Baking soda is an alkali: It raises the pH of the dye bath. While this can be helpful in some dyeing processes, it won't actually fix the dye to the fabric.

* Fixatives work by forming bonds: Fixatives like vinegar (acetic acid) or alum create chemical bonds between the dye molecules and the fibers of the fabric, making the color more permanent. Baking soda doesn't have this capability.

Instead of baking soda, you should use a proper fixative like:

* White vinegar: For most natural dyes and some synthetic dyes.

* Alum: A traditional mordant for natural dyes.

* Metal salts: Specific salts like copper sulfate or iron sulfate can be used for certain dyes to achieve different color shades and better permanence.

Here's a general process for dyeing with a fixative:

1. Prepare the dye bath: Dissolve your dye in hot water.

2. Add the fixative: Follow the specific instructions for your chosen fixative.

3. Immerse the fabric: Submerge the fabric in the dye bath and let it sit for the recommended time.

4. Rinse thoroughly: Wash the fabric with cold water until the water runs clear.

5. Dry the fabric: Hang the fabric to dry completely.

Remember to always follow the instructions for your specific dye and fixative.