It's because too many people were adding their own profit margin onto the postage costs for an item.
For example, someone might have offered an old postcard for sale, with bids starting from £3 but with £5 postage to be added on. Another seller might have been offering a similar postcard with bids starting from £5, but with only 50p added on for postage. The seller with the ridiculously high postage charges would gain an unfair advantage, because his item would appear first when 'lowest to highest' price was selected by potential bidders.
So eBay now force sellers to include postage and packing within the price that the item is offered at. (i.e. you have to set your 'minimum bid' figure at a level which includes your anticipated expenditure on p&p).
Chris