Sessions (everyone should despise that racist elf) will try to have the federal government enforce the illegality of it while states that have it legalized will probably unify and fight against it in the courts due to how much of a profit it's making for said states.
The problem is that it can never actually develop into an industry while it's federally blocked. If it's still illegal under federal law, that means that banks won't touch any of the profits or payroll, meaning it's a strictly cash business. That leaves so much room for corruption and crime and that will just be used on a federal level to further justify the legality of it. They need like a state-run pot money co-op or something, but that will probably not happen until the business proves to be profitable and crime-free.
What is the reason that it is federally illegal? Have they given a reason for it?
Not that I'm aware of, at least nothing beyond the standard "It's a drug, drugs need to be controlled substances because it's dangerous to allow people free reign over drugs" idea. In some cases that has some merit, but not so much in pot's case.