Why not?  Simply because Taiwan is an inseparable part of China, i.e. China's territory.  No further explanation is needed.

Recently US senate passed a proposal, suggest the administration let US battle ships anchor at the harbors of Taiwan.  I don't think Trump administration would dare follow this kind of suggestion to compromise its national interest by doing a silly thing like that.  Time was when crisis of 1996, US carriers had to stay away from the coast of Mainland, as well as from Taiwan, at least 200 miles away.  Could you imagine America would be so stupid to let its battle ships like sitting ducks in Taiwan's harbors as fall prey to PLA's missiles(or rockets) now in 2017?  In fact, unlike blocking USSR during Cold War mainly with its economic means, America now is facing not only a military powerhouse, but also an economic and financial giant: China.  Trump will surely think twice before signing the document.   

China may tolerate Taiwan Relation Act for the time being, but don't you forget, Taidu advocates, she will execute her Anti-Secession Law anytime she thinks fit.