Immigration is outlined in the constitution, is it not?
Yes and no. However, conceptually, I think immigration is clearly a federal issue. Conversely, I think legalization of marijuana fits squarely within states' police powers.
The Constitution never uses the word immigration, so how is it that the rules for immigrants, and quotas for countries, are set by the federal government and not by the state governments? After all, as the 10th Amendment states, are the powers not delegated to the United States held by the states, or the people?
The Supreme Court has ruled that the Congressional power to regulate naturalization, from Article 1, Section 8, includes the power to regulate immigration (see, for example, Hampton v. Mow Sun Wong, 426 U.S. 88 ). It would not make sense to allow Congress to pass laws to determine how an immigrant becomes a naturalized resident if the Congress cannot determine how, or even if, that immigrant can come into the country in the first place. Just because the Constitution lacks the word immigration does not mean that it lacks the concept of immigration.
There is also an argument that immigration is an implied power of any sovereign nation, and as such, the federal government has the power to regulate immigration because the United States is a sovereign nation. While it is true that the United States is a sovereign nation, and it may be true that all sovereign nations have some powers inherent in that status, it is not necessary to determine if immigration is such a power that does not even require constitutional mention, because the Naturalization Clause handles the power.