Famine, disease, displacement. The Taiping Rebellion went on for fourteen years and affected a large swathe of territory.

Also keep in mind that these so-called "death tolls" are only estimates. Most of times, historians rely on census data to calculate death tolls. For example, the Tang's An-Shi Rebellion has a highest estimated death toll of 36 million, but this was largely based on census data taken before and after the rebellion. However, many people displaced from their homes were never counted by the census, so it appeared that the Tang experienced a massive population decline, when in fact, many people simply went somewhere else.

The fastest answer is twofold; the nature of the deaths, and who died.

Events like the Taiping Rebellion happened across the world, at a time when Orientalism ran supreme. I'm currently working through some microfilm copies of the San Fransisco Chronicle, and the Taiping Rebellion doesn't get front-page news in any of the 1852 pieces I've read. (Though it might appear in other dates.)

The second reason is simply the nature of the deaths. The Taiping Rebellion was an internal struggle, with many of the deaths happening as a result of famine and 'natural disasters' like flooding rather than actual, physical massacres like the Holocaust.