Why haven't I received a decision letter yet?

I applied to NYU New York (first choice) and NYU Shanghai (second choice). I got an acceptance letter to NYU Shanghai on March 28, but I haven’t heard anything back from NYU New York. I am supposed to receive a decision by April 1. If I do not get a decision from NYU New York soon, does that mean I am rejected? Does being accepted to Shanghai disqualify or interrupt my acceptance to New York?

Did you check admissions portal? https://www.nyu.edu/admissions/undergraduate-admissions/access-your-admissions-decision.html

It means that you are rejected of NYU nyc if they accept you for NYU Shanghai

Exactly what @burgerdebambi said. You will not receive a separate letter.

Somewhere in your acceptance letter it says “please be assured that we considered you for all of the selections you made on your application before extending this offer”.

Yep, you were accepted to Shanghai not NYC. Which can be a blessing in disguise; the Shanghai program is beautiful and very well respected globally