Because the Battle of Rzhev didn't change the war, Stalingrad did.
Both featured overextended German salients attacked by large Soviet forces. In both cases the Germans decided to hold their exposed positions for a future attack rather than withdraw to safer lines. The Eastern Front was already littered with many such battles, Demyansk and Kholm for example; overextended German armies temporarily surrounded throwing off a Soviet counter-attack. But in all the other battles the Germans won. Stalingrad was different because this time the Soviets won, and won so completely.
Operation Uranus was a turning point. It was the first time a German salient had been successfully cut off, surrounded, and destroyed on the Eastern Front. The Germans were overextended, vulnerable in the flank protected by weak satellite armies. German (mostly Hitler's) overconfidence and purely offensive mindset could be exploited: rather than withdrawing when their flanks collapsed the Germans would remain in position confident they could lift the siege and continue their attack.
But it didn't work out that way. The German 6th Army was destroyed. The 4th, and two Romanian armies, were mauled. They represented the destruction of a large chunk of the German strategic offensive capability in the south.
Stalingrad would represent the high water mark of the German invasion of the Soviet Union. Whereas Rzhev was another failed Soviet counter-offensive.
And because, sadly, the West is largely ignorant of the details of the Eastern Front. My own knowledge is a vague timeline punctuated by major events. If you asked most Westerners about the Eastern Front they probably have a vague idea it happened. If they could name any specific battles it would be famous sieges of Leningrad and Stalingrad, the skin-of-their-teeth defense of Moscow, and the crushing victory of Barbarossa. That's about it.
Stalingrad is well known in the West because it got a big budget Western movie raising the Western public's interest in the battle. Unfortunately it was a mediocre and wildly inaccurate movie based on a wonderful book.