The Appeal Court hearing is January 9, 2024. SCOTUS may have wanted more discussion on this issue before making the final ruling. Instead of just doing the easy task of deciding this particular case, SCOTUS often attempts to develop legal tests that can be applied to strange hypotheticals. Considering all possibilities takes time.

The easy way would be to consider that only one former U.S. President has ever been criminally indicted. Trump is an outlier. SCOTUS could limit its ruling to ONLY TRUMP, only consider the facts in this case, and say future Presidents will get their own chance in court if needed. But that would be too easy.

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USCA Case #23-3228 Document #2031216 Filed: 12/12/2023
Trump's argument is immunity is needed to prevent "former President may be prosecuted for ... using lethal force abroad."
"Under Former President Trump’s View Of Absolute Immunity, A Future President Could Disregard Current Criminal Restraints Against Using The Military To Alter Election Results."
storage.courtlistener.com: PDF document

"In a 5–4 decision, the Court ruled that the President is entitled to absolute immunity from legal liability for civil damages based on his official acts. The Court, however, emphasized that the President is not immune from criminal charges stemming from his official or unofficial acts while he is in office."
Wikipedia: Nixon v. Fitzgerald - Wikipedia

"Courts traditionally have recognized the President's constitutional responsibilities and status as factors counseling judicial deference and restraint... When judicial action is needed to serve broad public interests--as when the Court acts, not in derogation of the separation of powers, but to maintain their proper balance, or to vindicate the public interest in an ongoing criminal prosecution--the exercise of jurisdiction has been held warranted. In the case of this merely private suit for damages based on a President's official acts, we hold it is not."
edge.sagepub.com - Nixon v fitzgerald

Past presidents, while never indicted, have faced legal woes of their own
Nixon was an "unindicted co-conspirator"
"Clinton was accused of giving false testimony to a grand jury, which amounts to perjury, and obstruction of justice."
A number of presidents have been accused of being war criminals, but these "did not produce any movement toward prosecution in the U.S.".
npr.org - Past presidents while never indicted have faced legal woes of their own