Because the "left" is taken over by pseudo left
Pseudo left: you know, the Wu you Zhi xiang crowd.
If the left (as an ideological force) fails to present viable solutions to current problems and keep hacking back to "good ole days" to arouse sympathy and support, sooner or later they will be irrelavent. Because at end of day people want is solutions to their problems and hope for a new future, not just reminscent of what was. If "Left" has any chance to regroup, it must stop these ludicrious shouting from the "Pseudo left" , and start to develope a coherent, forward looking (i.e. don't rely on the "good ole days " and "China is doomed, current govenment is rightist" etc messages), positive messages and solutions.
The "Left" as a collective have to face the fact that it doesn't help you to keep arguing "cultural revolution is Good, Anti-Right is not out of proportion, Famine didn't exist" etc. because they are simply not facts that can not stand on themselves.
Moderate Left can not survive on the "left" if the "left"'s voice is drowning out by these ridiculous noises.
To win people over, you have to present Clear, Rational, arguments. I don't see the Wu You Zhi Xiang crowd doing that. I only see vicious attacks backed up with half ass arguments against the so called "Right". and the only people who got hurt is the Moderate Left. who happen to be tring to govern. because alot of Moderate Left's energy is sapped by trying to combat these shoutings, because if they don't it makes the whole "Left" looks like a bunch of radicals.
Unfortunately moderate left is under assault from both Pseudo left and Right. If the Pseudo Left people want the center of politics in China to lean left, it better stop attacking the moderate left, it will only drive majority into the right.
btw, I see a strong parallel between the way "Left" in China is going and Republican Party in US is now.
I see CCP in current form as a fundamentally a Moderate Left Force, and is governing as such. Government has corruption. Toleration of Corruption does you no good in PR.