Why can't a state be expelled? Where is that forbidden by the Constitution?
The powers granted to the federal government are delegated, or enumerated (see the 10th Amendment). The federal government can constitutionally do only those few, specific items delegated it by the States. They can print money, maintain the navy, deliver the mail, etc.
The Constitution does NOT limit the States in any way whatsoever. Nor does it give the federal government any authority over the States. The Constitution (1) defines the structure of the federal government, (2) lists the few powers given, or enumerated, to it (about 24 specific powers), and (3) list a few of the rights which NO ONE can ever limit, or take away, in the bill of rights.
Again, the federal government is a servant of the states, not the master. When it ceases to serve the States, the States can end the contract, called the Constitution.