Given the obvious similarities between Beijing and Cairo, what accounts for the difference in the reaction to the two cases? Here are a few explanations:
The Tiananmen Uprising arose suddenly, while Egypt was smoldering for two years.
Although Chinese cities had engaged in scattered anti-government protests since the mid-1980s, the cause of the 1989 conflict was the sudden death of Hu Yaobang, a popular refomist politician, that April. Public mourning for Hu morphed into a sustained movement against Party rule, with students and urban workers alike demanding political liberalization. Ultimately, Deng Xiaoping decided to turn the military against the protesters on the night of June 4th, and before long China basically returned to normal. The entire crisis lasted for six weeks.
Egypt, on the other hand, has experienced non-stop instability since Hosni Mubarak resigned as president in February 2011. The election of Mohamed Morsi as president -- hailed worldwide as a major democratic breakthrough -- only exacerbated existing political divisions within the country and ultimately forced the Egyptian military to overthrow Morsi in a coup.
While this week's killings mark the worst episodes of violence in Egypt's post-Mubarak era, they reinforced the sense that the country is fundamentally unstable, and such incidents aren't terribly surprising.
China's protesters were more sympathetic than Egyptian ones.
Not all of the Chinese who massed in Tiananmen Square wanted to overthrow the Communist Party; many, in fact, sough more modest reforms like being able to choose a profession. But Westerners idealized the protesters as liberal democratic revolutionaries, a sentiment that only grew when protesters built an crude copy of the Statue of Liberty in the square. To more seasoned observers, China's youth weren't an existentialist threat to the state, and as Sam Crane writes, the crackdown was "an example of a very firmly rooted regime employing a small fraction of its well organized repressive capacity to put down a limited protest that did not fundamentally threaten the bases of state power." The outrage over Tiananmen wasn't just over the numbers killed; it also reflected a belief that the tragedy was avoidable, a belief held by China's then-premier Zhao Ziyang, whose opposition to the crackdown resulted in a subsequent purge and house arrest.
Meanwhile, Egypt's protesters are demonstrating in support of a politician, Mohamed Morsi, who fits nobody's idea of Thomas Jefferson. Despite receiving only limited support from Egypt's voters, Morsi acted as if he held a broad mandate, in effect moving to undermine democratic institutions in order to entrench his and the Muslim Brotherhood's hold on power. Morsi's base of support also included Egypt's Islamist population, whose illiberal views on politics and religion put off would-be Western sympathizers. Whatever their other merits, Egypt's pro-Morsi protesters are demonstrating to restore a president who had failed to govern effectively in the first place. China's demonstrators, by contrast, were untainted by an association with an ineffective regime.