People see someone making $6/hour and think that passing a law that minimum wage must be $8 means that person will now make $8, but technically the law only says "it is now illegal to pay that person $6."
Some businesses may be able to afford the raise and still turn a profit, but for others, if that person's labor is worth $7/hour to your bottom line, it is suddenly no longer a good business decision to keep them employed.