為何 so2 不能decolourize acidified potassium dichrome?

The bleaching action of sulphur dioxide is due to that sulphur dioxide is a reducing agent. Only some dyes (NOT ALL dyes) can be bleached by sulphur dioxide if they are reduced by sulphur dioxide to give colourless products.
SO32-(aq) + dye → SO42-(aq) + [dye - O]
where [dye - O] is colourless.

Acidified dichromate ion can also be reduced by sulphur dioxide. However, the dichromate ion is reduced to chromium(III) ion which is green in colour. Therefore, the product will not be colourless/white.
Cr2O72-(aq) + 3SO32-(aq) + 8H+(aq) → 2Cr3+(aq) + 3SO42-(aq) + 4H2O(l)