If the federal government had the power to prohibit sales of marijuana, then there would have been no need for the 18th amendment (which prohibits the sale of alcohol,and was repealed by the 21st amendment).
There was no need for the 18th. The temperance reformers thought it would be harder to overturn than a federal statute.
Once the 18th was passed, the 21st was necessary to repeal it. Read Section 2 of the 21st amendment -- it turns the legalization power over to the states from the federal government.