You would be happy reserving new territories ONLY for Democrats? The gist of the decision is that all members of the union have equal rights to the territories. Seven justices agreed. Sorry if you have a problem with that.
Six justices (Taney, Catron, Cambell, Daniel, Grier, and Wayne) held that the slavery restrictions in the Missouri Compromise were invalid, but there was no consensus on why. Each justice submitted a separate opinion. Taney's discussion regarding the territorial question took about 21 pages. (This question is thoroughly reviewed in Chapter 16 of Fehrenbacher's case study.)
10 of Taney's 21 pages on the Territorial question were dedicated to the odd idea that the "Territory clause" in the Constitution applied only to "land owned or claimed by the United States in 1789. In all areas subsequently acquired, such as Louisiana, the law had no force of any kind." As Taney wrote, the Territory Clause "was a special provision for a known and particular territory, and to meet a present emergency, and nothing more." That interpretation did not have the support of the majority of the Court, as three other justices reasoned differently on the question, coming to the same conclusion with regard to its validity. Taney's odd construction of that constitutional clause has never been upheld by later Courts.
The power of Congress to regulate and make rules for territories was already established before the Constitution was proposed for ratification. The context of the Territory Clause was readily apparent. Taney chose to misrepresent the history and misinterpret the original meaning of the Clause to suit his own activist purposes.