Because the ninth amendment is a restraint on government and the modern left in cluding modern liberals believe gov't imposes people's rights on them, rather than them being natural.
The assumption is the existence of rights in the natural law and the common law. The Bill of Rights therefore is not an exhaustive list of rights, and that is the point the 9th amendment makes. Madison would not support a bill of rights without this, since he was afraid the government would expand it's powers up to the point where something wasn't explicitly prohibited for the government to do. And that has basically happened, even with the 9th amendment. Modern liberals have no interest in advocating adherence to it, since it would call into question everything the government has done in terms of abrogation of property rights or other regulations that rest on an unconstrained view of the enumerated powers.
Natural law is a foolish myth and common law only comes to exist through the legislator's action, so that any right existing in common law is a right granted by the legislator. So this only makes it clearer why the 9th Amendment is an empty shell.
Well, sure, if you don't believe in the ideological basis for the American Revolution and the Bill of Rights it's not going to make any sense.
Well, if this is the case, the Bill of Rights itself does not believe in the "ideological basis for the American Revolution and the Bill of Rights". Why in the world would you need to enumerate a set of rights in your constitution if those rights are "natural" and exist independently from any legal document?
As a protection for certain of those rights that they believed would be most likely to be infringed by the national government.
So I guess they were just too lazy to include the other "natural" rights into the list as well.
They were trying to be forward looking and not limit the scope of the rights being protected from the gov't intrusion and usurpation.
