There are a few factors that led to Auschwitz being better known than the other Nazi extermination camps. The three most important ones were the large number of victims, the fact that the camp was liberated virtually intact, and the large number of survivors. Other similar camps aren't as well known because they lack one or more of those three characteristics.

Starting with the most obvious of these, Auschwitz had the highest number of victims of any Nazi extermination camp, with approximately 1.1 million victims; around one sixth of the victims of the Holocaust died at Auschwitz. After the war, various estimates were presented; a figure of 2 million was commonly cited, and some estimates went as high as 4 million victims. The commandant of Auschwitz for much of its operation, Rudolf Höß, was accused of being responsible for 3.5 million deaths at his interrogation in 1946, and famously replied that he was only responsible for the 2.5 million people killed in the gas chambers, with the rest being from starvation and disease (he estimated about 3 million victims). At his trial in Poland later that year, he stated that he thought 2.5 million was probably too high, and historical research eventually showed that that was true. Franciszek Piper calculated a death toll of 1,082,000, which as generally become the accepted figure.

However, having a large number of victims didn't necessarily assume persistent notoriety for a camp. Most of the other extermination camps aren't well known among the general public. For example, Treblinka had between 875,000 and 925,000 victims, and Belzec may have had as many as half a million, but these camps don't carry the same level of infamy despite their huge death tolls, so it's clear that that factor alone isn't enough.

The second thing that distinguishes Auschwitz from the other extermination camps is that it was captured mostly intact. Killing operations at Auschwitz continued until the end of October 1944, much later than the other extermination camps. The Germans did make some efforts to conceal their actions, destroying some of the gas chambers and crematoria, but Soviet forces advanced quickly enough that the Germans weren't able to totally cover their tracks, and the liberators were able to directly witness the fact that it had been set up to kill people on an industrial scale.

The only other extermination camp that was captured largely intact was Majdanek, which had the smallest killing facilities and the lowest death toll of the six main extermination camps (approximately 58,000 victims). Belzec, Sobibor, and Treblinka had all been demolished at the conclusion of killing operations in those camps in late 1943 and early 1944. Killings continued until January 1945 at Chelmno, but the camp had been fully destroyed before the Soviets arrived (and that camp had used mobile gas vans to kill people, rather than permanent gas chambers and crematoria). The Germans undertook a program known as Sonderaktion 1005 in 1943 and 1944 in order to disguise the evidence of the Holocaust as the war turned against Germany; this included burning buried corpses, grinding up bones, and planting trees over the sites of the former extermination camps. This effort wasn't totally successful, of course, but it did diminish the impact of the liberation of the fully dismantled extermination camps.

Finally, Auschwitz had more survivors than all of the other extermination camps combined, approximately 200,000 people. This is due to both the fact that the camp operated for far longer than the other extermination camps and the fact that Auschwitz wasn't purely an extermination camp; Auschwitz II-Birkenau (the extermination camp) was part of a much larger complex that included both the original concentration camp (Auschwitz I) and a massive industrial labor camp (Auschwitz III-Monowitz), both of which were also liberated mostly intact. As a result, there were a large number of survivors from Auschwitz I and Auschwitz III, as well as those who escaped or survived the death marches from Auschwitz II, who were able to testify to the camp's horrors.

By contrast, most of the other extermination camps had almost no survivors; Treblinka had 67, Belzec had 58, and Sobibor and Chelmno had seven each. As a result, there were far fewer people to testify and write about those camps, decreasing the amount of public awareness. These camps have had some presence in the media and popular culture (e.g. Treblinka through the film Shoah and Sobibor through the film Escape from Sobibor), but they haven't had the extensive cultural presence that Auschwitz did, thanks to prisoners who became famous for their writings about the camp (Elie Wiesel, Primo Levi, etc.). Auschwitz is also the only extermination camp where the killing operations were directly photographed.

Historical memory is always a complicated thing, even more so when you're talking about the memory of the general public rather than historians. Auschwitz certainly deserves the notoriety and popular attention that it's received, but it's unfortunate that other sites of industrialized mass murder like Treblinka and Belzec remain largely unknown. (As an aside, I've experienced a similar phenomenon in my recent work on Soviet POWs; most people are entirely unaware of their experiences in Nazi Germany despite the fact that they were the second largest Nazi victim group.) Given the combination of factors I described, though, it's easy to see why Auschwitz has become the predominant symbol of the Final Solution.

Sources:

Yitzhak Arad, Belzec, Sobibor, Treblinka: The Operation Reinhard Death Camps (Indiana UP, 1999)

Debórah Dwork and Robert Jan Van Pelt, Auschwitz (W. W. Norton, 2002)

Franciszek Piper, "Establishing the Number of Deportees to and Victims of the Auschwitz-Birkenau Camp," Yad Vashem Studies XXI (1991): 49-103

Laurence Rees, Auschwitz: A New History (Perseus, 2005)