The joke is that 9/11 was blowback from decades of US meddling in the middle east, especially it's support for Saudi and it's actions in Afghanistan. While the specifics weren't readily predictable a major attack on the US from al-Qaeda or affiliated groups was inevitable. They'd already tried to blow up the WTC once in I want to say 1994. The US then used this blowback to gut it's own civil rights, re-structure it's government along more explicitly fascist lines, and launch a totally unrelated war to pillage Iraq that murdered a million people an destabilized the region. The GWOT was used as a blanket excuse for regimes all over the world to pass sweeping new police state powers and suppress or exterminate challenges to their power under the pretense of counter-terror operations. The US officially made torture legal, crushed numerous domestic pro-democracy movements, and diverted trillions of dollars to the MIC.

Basically the whole situation is so utterly, madness-makingly absurd what can you do except joke about it? It changed the course of the history of the world, touching almost every country in the world, highlighting the naked hypocrisy of The West, strengthening many of The Wests enemies in entirely predictable ways, and slaughtering vast numbers of innocent people totally unrelated to the nominal causus belli. If it hadn't actually happened you'd only expect to reach about it in something like a Pinchon novel.

There's a lot of existentialism, nihilism, and absurdism on the left, given it's broadly materialist grounding and tendency towards athesim or agnosticism. Humor is one of the main ways to cope with the inherent meaninglessness of existence. Joking about a US tragedy also undermines the power of US propaganda - By mocking the event we deny the religious, sacred significane placed on it by the US's twisted American Civic Religion.