Because nobody's even hinted that this has been done before. When someone's comments are edited, that person can see it[1], as can anyone who remembers the comment pre-edit. Add to that the fact that comments that are worth editing (beyond the rather unimportant "fuck u/spez" type of comment) are also presumably highly visible, it would be pretty hard for them to have a history of editing comments without someone having noticed before now.
[1] Modifying Reddit software to let them hide the edit from the comment author would be visible to anyone with source access, meaning it can't be done without the support of all of their developers, and so probably wasn't.
Reddit's website source code is actually open sourced. The backing database is not though.
And you know this how, exactly? Because the perpetrator said so?
I find the scenario that this was the first and only time this was done implausible.