> Cannabis isn't illegal everywhere.
In the US it is, outside of the US, it may not be.
> Washington and Colorado legalized it recently, and other states will likely follow in the future.
A number of other states already have legalized it for certain uses, but no state level action will change that it is illegal under federal law, and the feds have, including under the current administration, carried out enforcement actions against operations that were legal under state laws. There've been a few statements to the effect that this is not a priority, but its still part of the legal landscape.
National legalization, even if for medical purposes only, would open up a clear space for startups, but short of that any startup is creating a massive paper trail of evidence for conspiracy to violate federal narcotics laws (which are expressly included within the scope of the RICO statute as predicate offenses), and both the companies and individuals involved are risking criminal and civil prosecution under both narcotics and RICO laws as well as potential civil forfeiture actions against the assets that are instrumentalities of the legal violations.
All that on top of the usual business risks that come with operating a startup.