Why do solo travellers so often have to pay more, what's the justification for single supplements?
Hotels have traditionally provided services on the basis of people travelling in pairs, with a limited number of single rooms. Since hotels charge per room, not simply the number of people staying in them, solo travellers will proportionately pay higher prices, though many hotels will offer a discounted rate to solo travellers rather than charging them the full room rate.
Even when single rooms are available they're not generally half the size of a double and also will usually have en-suite facilities, so consequently hotels will not usually charge half the rate they would for a double.
To keep costs down, travel out of the peak summer months as rates are much lower and hotels will be more likely to offer discounts to solo travellers.