Tibet is absolutely crucial for China Yangtze and Yellow River originates there not to mention the elevation of the plateau anyone controling the region can pretty much do deadly artillery strikes against China's heartland the same in the case of Aksai Chin it's the only place that connects Xinjiang and Tibet, That is not the case of Arunachel it's not a crucial land mass for China in fact Chinese leadership only started to claim the area as a way of leverage in negotiations with India for Aksai Chin, Both countries has their own reasons, For India their territory was determined by Britain, Aksai chin and Arunachel were part of British India, Tibet on behalf of Qing empire waged war but to no avail even resulting in De facto Tibetan independence from Chinese crown Qing court was smart enough to adopt a passive stance and not sign anything with the Brits regarding formalization of border to have plausible deniability in the future, For China Tibet was a part of China and was annexed from it by Imperial powers when it was at its weakest and regaining the territory is non-negotiable to end it's Century of humiliation and rejenuvate China's old glory, it's geopolitics and a clash of interests, personally I would have preferred India formally recognise Aksai Chin as part of China in return for Burang county (Mount Kailash) in the 50s and ended it once and for all
tl;dr damn brits.
tl;dr damn brits.