Actually there have been many attacks, but not since Xinjiang turned into a total surveillance state (ironically it ended up as a 'successful' trial run, parts of which have since rolled out nationwide).

Open question: What would you do if you have increasingly frequent terrorist attacks in major cities year after year at the same time that Wahhabism is taking root in your previously tolerant community? China doesn't have the luxury of two oceans to protect itself from religious fanatics. Xinjiang itself was Buddhist and Indo-Persian in culture well into the medieval period, until conquered and forcibly converted by Turco-Islamists who reveled in "shitting on the Buddha's head" and destroyed many monuments -- basically the Taliban and ISIS. Since then, Xinjiang's Islam has mellowed out but the insidious and fanatical influence from West Asia continues nonstop.

It's absolutely defensible and moral to stop any fundamentalist backsliding in Xinjiang or indeed anywhere in the world. Modern day Xinjiang's problem is very simple, it's one of people losing their identity in a modernizing world. You see the historically more Chinese-influenced Northern Xinjiang more peaceable and prosperous, and religion more tolerant and tolerated. The opposite can be said of Southern Xinjiang. People who fear change tend to become reactionary and turn to religion. It would have happened regardless of whether they were in China. The alternative is to never develop like some of the Central Asian -stans (e.g. bride kidnappings in Kyrgyzstan), but then who wants to live there -- not even Uyghur émigrés, who never go to those places but always head for the EU, the US, or if they have no choice, Turkey. It's sad to say but it might take materialist brainwashing ('re-education') to reverse religious brainwashing.