Earlier quoted context omitted.
Because the poor and working class spend all their money on necessities and in doing so incur higher effective transaction taxes relative to their income.
Does differential tax rates on necessities Vs luxuries help here? For example, in the UK, sales tax is discounted or not applied on many types of food, children's clothes, etc. - yes, you end up with strange arguments at the edges (a famous one being the lawsuit to decide whether Jaffa Cakes are legally cakes or biscuits) but I think it _could_ work.
We had a lawsuit to determine if KitKat is a chocolate or a biscuit for similar reasons:
https://www.businessinsider.in/KitKat-is-a-biscuit-and-Parac...
> In the case of Nestle India Ltd Vs Commissioner of Central Excise, Mumbai of 1999, it was ruled that KitKat is a biscuit and not a chocolate that is taxed high.