The answer is because a 50% discount from a 400% markup is no discount at all. Diamonds simply do not cost as much as you pay for through retail sellers. They are often 400% cheaper.
The diamond cartel purchases diamonds in rough all over the world but get this...they don't sell them! They horde them in huge stockpiles and release the select rough to various "Insiders". That's how they control prices and can deliver such an outrageous markup to you.
Retail store owners have enormous overhead and they cloak their greed by crying overhead cost as the reason for diamond prices. Overhead is the cost of doing business which includes rental space, utilities, stocking, advertising, salaries, insurance, etc.
The truth is that diamond dealers buy stones at 400% cheaper than the public. They charge enormous markups to pay their overhead and make a profit. You could avoid the enormous markup by purchasing your stones from us because we don't have to worry about a huge overhead.