Because the purveyors of diamonds, and you could argue jewelry in general, have positioned themselves as proxies for love and sex. Those are the things we value and the reasons we spend money, but we're not allowed, either by societal convention or law, to spend money on them directly. But we're allowed to spend exorbitant amounts of money on a glittery status symbols that serves as a measure of devotion. It's not the rock itself that's important, it's the willingness to sacrifice a substantial amount of money.
So if the point of buying a diamond is to show your love for someone by sacrificing a meaningful amount of money, it becomes important to distinguish between stones that someone actually sacrificed a substantial amount of money to purchase and stones where someone didn't sacrifice as much.
We'd do better to address the insane custom of needing an expensive proxy for love. Think about it...if the custom were for a man to take two months salary, withdraw it from the bank in $100 bills, drop to one knee and present it to his girlfriend while asking her to marry him, we'd find that insulting. It would be like he's paying for love/sex, which our society deems dirty or objectifying to women. But substitute a glittery rock mounted on a hunk of metal, both largely obtained by heaping further misery on various third-world locations, and we somehow find that acceptable? Accepting the money would make her a whore, but accepting the ring just means her boyfriend loves her?
It's illogical and hopefully more women start making it known to their boyfriends that the ring isn't necessary and all that extra money, if it needs to be spent on something impractical, can be spent on making the wedding just a bit more lavish.