Seventy years ago China's claim was completely hollow and nonissue because china lacked the means to enforce the claims, there was no reason to make an issue out of such a nonissue.
Now china has the means to enforce these claim against all comers except the US, and the US sees american ability to successfully push back on china over these claims to be the litmus test all the regional powers will use to evaluate whether the US is a sufficiently effective and persistent security guarantor worth cultivating at the cost of annoying the much nearer china.
Basically, if America can't successfully push back on china in SCS, then the customary American role as the regional hegemony is at an end because countries will defer to china where Chinese and American interest collide.
Now china has the means to enforce these claim against all comers except the US, and the US sees american ability to successfully push back on china over these claims to be the litmus test all the regional powers will use to evaluate whether the US is a sufficiently effective and persistent security guarantor worth cultivating at the cost of annoying the much nearer china.
Basically, if America can't successfully push back on china in SCS, then the customary American role as the regional hegemony is at an end because countries will defer to china where Chinese and American interest collide.