Because there is already a slate of electors chosen, by the voters, according to law. The legislature would be acting contrary to the law by appointing a second slate. It's one thing if a court says that's the option they should follow. It's another thing for them to do it just because the decided to. No court is going to allow that. It opens up a scenario where any legislature in any state could override the election just because they don't like the results.
Whenever any State has held an election for the purpose of choosing electors, and has failed to make a choice on the day prescribed by law, the electors may be appointed on a subsequent day in such a manner as the legislature of such State may direct.
Couple this with § 5.Determination of controversy as to appointment of electors:
If any State shall have provided, by laws enacted prior to the day fixed for the appointment of the electors, for its final determination of any controversy or contest concerning the appointment of all or any of the electors of such State, by judicial or other methods or procedures, and such determination shall have been made at least six days before the time fixed for the meeting of the electors, such determination made pursuant to such law so existing on said day, and made at least six days prior to said time of meeting of the electors, shall be conclusive, and shall govern in the counting of the electoral votes as provided in the Constitution, and as hereinafter regulated, so far as the ascertainment of the electors appointed by such State is concerned.
If court challenges persist beyond the safe harbor date, then what? What if the PA Supreme Court behaves like the Florida Supreme Court and starts making ad hoc rulings against Trump that are not consistent with laws enacted prior to the election? What if a series of rulings shows a partisan bias that belies the facts presented to the court?
If a state legislature decides that §5 has not been met and offers its own slate, wouldn't a court (perhaps the Supreme Court) recognize, at this point, the state legislature's Article II Section 1 power as codified in US3 Chapter 1 §5 to let Congress sort it out?
-PJ