Most non-academic history book I've read say something about the land is mostly grass, not suitable for agriculture. In my university biology class, it says the land north is the best biome for agriculture, same biome as the one in mid US and Ukraine. Is there any limitation such as technology that made Chinese think it is infertile?
Instead of why they couldn't, I think this question is asking why they didn't care to. Short and simple, the answer is Chinese elitism. Much like ancient Greeks (save Alexander who was Macedonian anyway) the Chinese considered themselves the apex of human civilization. Surrounding territories were barbaric, underdeveloped, and wouldn't benefit them a whole lot. I think most people will remember the hubbub a few years ago about claims of the Chinese "discovering" North America before Columbus. Without getting into that specifically, the Chinese launched a massive exploratory expedition that sailed all around Southeast Asia and as far as West Africa. Paraphrasing and simplifying, the general consensus was "Giraffes are pretty cool but really we don't give a shit." Yes this one singular event, and yes it was one that happened much later in Chinese dynastic history, but it is indicative of ancient Chinese views vis-a-vis their neighbors. China's name, for instance, translated literally means "central country" highlighting just how self-important (though, largely because of good reasons) they were. Instead of thinking about conquering nearby people, the Chinese were significantly more interested in keeping them out.
It is important to note that two major Chinese dynasties were ruled by northern invaders so there are significant periods of time where China proper was attached to much more northern provinces. These invaders always ended up taking Chinese customs however largely abandoning/modifying their tribal identities.
The horse archers of the steppe nomads were a formidable fighting force. They conquered China much more frequently than China conquered them.
As long as the steppe archers had dominant military force they made it difficult or impossible for China to conquer the steppe, or to penetrate past the steppe and colonize and conquer the taiga to the North.
The people who learned how to conquer the steppe nomads were the Russians. They had already colonized across the Siberian taiga, all the way to the Pacific Ocean, untroubled by competition from rivals further south in Asia, because those rivals were cut off from the taiga by the steppe horse archers, and the steppe nomads were not interested in the forested taiga, as it had no grazing for herds and horses.
Once the Russians learned to conquer the steppe nomads, with a combination of guns and forts, they did (though it took them circa 300 years). The Russians had the means and the desire (because they were constantly being raided and enslaved by the horse archers) to conquer the steppes earlier than the Chinese. They did. The Russians were formidable enough that the Chinese (and Japanese) despite some skirmishes, never seriously challenged them for control of the steppes.