Because it was not a "war" which China declared.
In 1955, China boldly started building a road (roughly N-S) across Aksai Chin and they completed the road construction by 1958. As Nehru was on a grand “Non-Alliance” movement, “Panch-Sheel” and “Hindi-Chini Bhai Bhai” mode, Chou En-lai was confident that Nehru could be persuaded to accept it. Having constructed the road, the Chinese then stationed troops to protect it.
During this period, Chinese official maps were showing not only Aksai Chin (39,000 sq kms) but also the entire NEFA (83,000 sq kms) as parts of China. When India took this matter up, China just brushed aside by saying that these maps were from the old regime and they had not yet updated them!
Chinese meanwhile were willing to negotiate provided they keep the area under their control (that is, Aksai Chin) and India keeps the area under its control (that is, NEFA) with minor boundary adjustments. As this was not acceptable to India, it thought of a “Forward Policy” where Indian army was asked to establish army outposts all along the Chinese claim line and in some places inside Chinese claimed territory.
But with no other option & also not enough preparations, Indian army was ordered to retaliate. China was strong. Before that, in 1959 there was an internal rebellion against China in Tibet led by Dalai Lama. This was crushed by China and Dalai Lama had to escape to India in disguise via the Tawang tract. China demanded that India return Dalai Lama to them, but India refused.
While China maintained upperhand furing all these incidents before the war itself, the end result was no different.
Post war, China was condemned globally for the policy put forward, despite it's attempt to prove itself as not guilty of fighting the war. But other nations blamed China was fighting the war just to prove it's dominance. To reduce this blame game, China stopped giving this war any more importance whereas India for obvious reasons had to keep the talks about this war in check.