Quality has nothing to do with it. If they were of even higher physical quality than mined gems but still reliably discernible, they would not have a significant impact on the market.

Look at it this way, the kind of person that places a high emotional value on a diamond is not going to feel the same way about a $50 diamond as they would a $3000 diamond. It's all about signalling and an inexpensive, albeit high quality, diamond does not send the same signals.

Now... if they could produce large stones that were indistinguishable from mined ones then you'd have a crack at eroding the gem market. But then the diamond cartel would just enact some sort of global "Certified Blood Diamond" program to prop up the traditional prices.