I'll try to put this as straightforwardly as possible, but it's a meandering topic at best. The gist of what I'm referring to is found here[1], so you should probably stop and read that first. Past that, the relevant things are two main doctrines, the first is the 9th[2] and 10th[3] amendments collectively, and the other is "enumerated powers[4]".
So, the general principle is that 1) Congress doesn't have the power to do anything that they are not expressly authorized to do by the constitution, or that is not obviously implied by an enumerated power (for example, if I expressly enumerated you the power to apportion the jelly beans, it would be implied that you could count the jelly beans, audit the jelly beans, etc.) Because congress is not expressly enumerated any power to make marijuana illegal, the states (if challenged) could assert that they have neither an enumerated or implied power to do so, and the laws that they made to illegalize marijuana consumption were not constitutional to begin with. Under the interstate commerce clause[5], they may have the ability to 'regulate' the sale and/or distribution of marijuana, but banning it for private consumption may be a bridge too far considering...
The Ninth Amendment, which states "The enumeration in the Constitution of certain rights shall not be construed to deny or disparage others retained by the people," combined with the Tenth Amendment, which states "The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people," means that the powers of the federal government should not be used to take powers away from the citizenry, and that any powers not delegated to the federal government (expressly or implied) are reserved to the states or the people. It is this last clause that allows states to decline orders by the federal government where are not obviously granted those powers to the federal government by the constitution.
Put simply, the general position is that the federal government has supremacy on matters that the constitution says it should, but should defer to the people or the states on powers the constitution doesn't mention. Because the federal government doesn't have the power to ban marijuana, then the states could argue that, and assert their own soveriegnty on its regulation. Because the states would be granting more rights to the people than the federal government in this scenario, and because the burden of enforcing it would be exclusive to the individual states, they would have a pretty good claim on grounds of federalism.
So, the general principle is that 1) Congress doesn't have the power to do anything that they are not expressly authorized to do by the constitution, or that is not obviously implied by an enumerated power (for example, if I expressly enumerated you the power to apportion the jelly beans, it would be implied that you could count the jelly beans, audit the jelly beans, etc.) Because congress is not expressly enumerated any power to make marijuana illegal, the states (if challenged) could assert that they have neither an enumerated or implied power to do so, and the laws that they made to illegalize marijuana consumption were not constitutional to begin with. Under the interstate commerce clause[5], they may have the ability to 'regulate' the sale and/or distribution of marijuana, but banning it for private consumption may be a bridge too far considering...
The Ninth Amendment, which states "The enumeration in the Constitution of certain rights shall not be construed to deny or disparage others retained by the people," combined with the Tenth Amendment, which states "The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people," means that the powers of the federal government should not be used to take powers away from the citizenry, and that any powers not delegated to the federal government (expressly or implied) are reserved to the states or the people. It is this last clause that allows states to decline orders by the federal government where are not obviously granted those powers to the federal government by the constitution.
Put simply, the general position is that the federal government has supremacy on matters that the constitution says it should, but should defer to the people or the states on powers the constitution doesn't mention. Because the federal government doesn't have the power to ban marijuana, then the states could argue that, and assert their own soveriegnty on its regulation. Because the states would be granting more rights to the people than the federal government in this scenario, and because the burden of enforcing it would be exclusive to the individual states, they would have a pretty good claim on grounds of federalism.
[1] - http://www.constitution.org/uslaw/16amjur2nd.htm
[2] - https://en.wikipedia.org/wiki/Ninth_Amendment_to_the_United_...
[3] - https://en.wikipedia.org/wiki/Tenth_Amendment_to_the_United_...
[4] - https://en.wikipedia.org/wiki/Powers_of_the_United_States_Co...
[5] - https://en.wikipedia.org/wiki/Commerce_Clause