Because with all the series that are abandoned, or completely untouched, it's stupid to double up work on a series. Really simple concept, champ. Don't know why I have to post this every time some speedscan reading mong goes "uh bluh duh me read thing same anyways why people no like dem!??" but for some reason incredibly basic, obvious ideas fly over the heads of morons.
While I don't care about dumb "unwritten rules" like "wait 3 months before taking up abandoned series" and "ask for permission" and so on, but sniping of regularly updated series is dumb for the reasons you mentioned, and if quality is lower it also degrades the experience for every reader with no upside, while original quality team gets demotivated.
This case however was far worse, because it was sniping with a chapter skip and lower quality of translation, so the important emotional moment of manga was spoiled by lack of context from previous chapter compounded by low quality, resulting in many people going "what did she mean? what happened?".
Not to mention, the update interface doesn't show if there is a chapter skip, so I went into that chapter all confused.
