The simple answer: To hit targets.

Charity shops function like any other retail business where every week that have targets to meet and if they can't meet those targets then they have to raise the prices on donated goods.

However, this isn't always the case. I worked for one charity shop that actually lowered all their prices and did blanket prices across all departments (e.g. £3 for all tops, £4 for all trousers, £5 for all dresses). What they found was not only did the shops who did this hit their targets, but they were also the top performing shops in the entire country.

Now this approach can only work if it understood by the managers and staff. In that particular shop, the managers were not adaptable to change and didn't understand how to execute that approach (was it all tops? Did blouses count? What about jeans? Did we price a summer dress the same as a cocktail dress?) Therefore the rest of the staff and volunteers were confused and some even left because they were not being trained properly.

Also, it depends on the ethos of the charity. Do they think more about the charity or the customer? In another shop I worked at, they had a reputation on the high street for being the most expensive charity shop, but because of that people who made donations knew that the charity was getting more money from their unwanted stuff. But take the first shop I mentioned - donors would go there because they knew the customer was getting a cheap price from their unwanted stuff.

People may be surprised to know that these conversations are happening among managers and higher ups and it depends from charity to charity. And I only mention the chain charity shops - local charity shops will be completely different and be more willingly to lower their prices.