Look at how similar the elevation map is of the region:
And that goes really well with Why Is It Colder in the Mountains Than at Sea Level
As tbb noted, the band near 30 north/south tends overall (climatologically) to be drier and hotter due to descending air caused by the Hadley circulation.
As to why this area stands out in comparison with other locations at the same latitude, the moisture in the air has quite a bit to do with it:
Today's dew points, courtesy of Pivotal Weather
Indeed, you can see it is fairly dry over parts of central and western China compared to many other places, such as eastern China and back nearer Europe... due in part to the affects of the mountains and distance from large bodies of water... though you may also notice other spots to the west do show comparable moisture for this date despite producing lower temperatures.
Ground cover differences are part of the reason (more plants = more area the solar energy is distributed into, plus typically store moisture, equating to slower temperature changes).
And then wind direction gives one final explanation why it's not as warm further to the west:
The northerly winds have been in place for a while back in Kazakhstan and Uzbekistan have brought slightly cooler temperatures down from the north, whereas the flow into China is less consistent and has only recently started shifting. Even in the summer, wind changes and even fronts are still often very important in the temperature layout.