Why? The US is crystal clear on the issue. While Congress has directed the Embassy be moved to Jerusalem (years ago, with Presidential discretion on the timing of the move), that's at the discretion of the supervising Ambassador, who prefers Tel Aviv.
H.R.1950
Millennium Challenge Account, Peace Corps Expansion, and Foreign Relations Authorization Act of 2003
Subtitle B--Basic Authorities and Activities
SEC. 221. UNITED STATES POLICY WITH RESPECT TO JERUSALEM AS THE CAPITAL OF ISRAEL.
(a) LIMITATION ON USE OF FUNDS FOR CONSULATE IN JERUSALEM- None of the funds authorized to be appropriated by this Act may be expended for the operation of a United States consulate or diplomatic facility in Jerusalem unless such consulate or diplomatic facility is under the supervision of the United States Ambassador to Israel.
(b) LIMITATION ON USE OF FUNDS FOR PUBLICATIONS- None of the funds authorized to be appropriated by this Act may be available for the publication of any official government document which lists countries and their capital cities unless the publication identifies Jerusalem as the capital of Israel.
(c) RECORD OF PLACE OF BIRTH AS ISRAEL FOR PASSPORT PURPOSES- The first section of `An Act to regulate the issue and validity of passports, and for other purposes', approved July 3, 1926 (22 U.S.C. 211a; 44 Stat. 887) is amended by inserting after the first sentence the following: `For purposes of the issuance of a passport of a United States citizen born in the city of Jerusalem, the Secretary shall, upon the request of the citizen or the citizen's legal guardian, record the place of birth as Israel.'.