Changing major in the graduate school
Today your interests lie in economics, but what if by the time you finish, you're interested in computational biology? Good luck selling Economics → Computational Biology to a graduate school admissions committee. On the other hand, with Mathematics as your base degree, you can cross over to most disciplines including Economics (with your chances boosted by further by internships in that discipline). 

The bottom line is: the narrower your major discipline, the fewer your opportunities.