Because we’re programmers, and programmers are infamous for being rules-/logic-driven.
If, as a comment below suggests, the joke is that it’s meant to be read in order 3, 1, 2, that violates the rule that race conditions typically don’t cause an entirely different program to produce the output. So if the joke is meant to be “lol we have a race condition”, bubbles should be mixed up for one person, not mixed between people.
People don’t get the joke because the joke violates its own internal rules.