Why not? Marshall wrote:
'If the act of congress gives this court the power to award a writ of habeas corpus in the present case, it remains to inquire whether that act be compatible with the constitution.'Article I, § 9 - of the Constitution, a legislative power which 'herein granted shall be vested in a Congress of the United States, which shall consist of a Senate and House of Representatives', explicitly states that the 'privilege of the Writ of Habeas Corpus shall not be suspended, unless when in Cases of Rebellion or Invasion the public Safety may require it.''... If at any time the public safety should require the suspension of the powers vested by this act in the courts of the United States, it is for the legislature to say so.'
The Act in question must conform to the Constitution. The Contitution states that SUSPENDING the writ of habeas corpus is a legislative power. It's not a case where the legislature must craft legislation to put the grant into effect, it's a case where the legislature must enact legislation to suspend it. If it has NOT been suspended, the courts could obviously issue a writ. Which is exactly what Marshall had to decide. He wrote, 'the power to bring a person up that he may be tried in the proper jurisdiction is understood to be the very question now before the court.'
Only by answering the question of whether or not the legislature had suspended the writ could that question be decided. So instead of obiter dictum, Marshall's statement that the suspension of the writ is a legislative power is the foundation for the decision, not extranneous to it. Only IF the writ had been suspended by the legislature could the court be denied the 'power to bring a person up' on a writ of habeas corpus.
Why not? Because the Supreme Court can only rule on issues brought before it. It cannot issue an advisory ruling on matters which had not happened yet.
The Contitution states that SUSPENDING the writ of habeas corpus is a legislative power.
No it doesn't. The Constitution only says that habeas corpus cannot be suspended except when in cases of invasion or rebellion the public safety requires it. It does not say that only congress can suspend it. That may be inferred since the restriction appears in Article I and you seem to believe that Article I only contains items concerning the legislature. But Article I also contains restrictions on the states. So the question of who may and who may not suspend habeas corpus is not specifically limited to the legislature, and the question of who may suspend it has never been addressed by the Supreme Court.
Only by answering the question of whether or not the legislature had suspended the writ could that question be decided.
Except that you need to read the entire decision, not just the parts that, taken out of context, appear to support your position. Ex Parte Bollman & Swartwout, as the Chief Justice pointed out, involved whether the power to award a writ of habeas corpus, in such a case as that of Erick Bollman and Samuel Swartwout, has been given to the Supreme Court. It did not involve who could suspend habeas corpus since it had not been suspended.
So instead of obiter dictum, Marshall's statement that the suspension of the writ is a legislative power is the foundation for the decision, not extranneous to it.
You are mistaken. Since habeas corpus had not been suspended by anyone at the time of Bollman and Swartwout it was not a question for the court to decide. The Chief Justice's comments were indeed in dicta, and were not legally binding.