why chlorine bleaches a substances permanently but sulphur dioxide does not?

Both sulphur dioxide and chlorine are bleaching agents. Both require moisture for their action as a bleaching agent. However, the bleaching action of chlorine is based on oxidation, while that of sulphur dioxide is based on reduction. Also, while bleaching by sulphur dioxide is temporary in nature, chlorine causes permanent bleaching. 

When chlorine reacts with water, it produces nascent oxygen. This nascent oxygen then combines with the coloured substances present in the organic matter to oxide them into colourless substances.

Coloured substances + [O] → Oxidized colourless substance

On the other hand, sulphur dioxide actions as a bleaching agent by reduction. It removes oxygen from the dye. However, the atmospheric oxygen slowly reoxides the bleached material, because of which the material changes back to its original colour. This is why bleaching by sulphur dioxide is not permanent.