Throughout time the desired vessel for wine has changed from ceramic clay, to concrete, to wooden barrels, to stainless steel. While these changes occurred, concrete continued to be used in renowned regions of the world such as Bordeaux and Burgundy. France's Nomblot may have swung favor back to concrete with his discovery of the egg.
As opposed to stainless steel tanks which swish fermenting juice with a left to right motion, concrete eggs swirl the juice. This is just one of the supposed benefits of concrete:
The walls of concrete tanks can be up to six inches thick, providing a measure of natural insulation from outside temperature swings that stainless steel tanks cannot provide without help, and since many winemakers prefer the wine complexity that develops during fermentation, concrete has definitely threatened stainless steel.