Why the market for a product might reduce in size?

The market for a product may drop because

• Changing times make the product less useful - e.g. buggy whips
• A better product that does the same thing comes along- e.g floppy disks sales drop because of flash drives
• A cheaper substitute that is good enough comes out-- e.g. oleo for butter,nylon for silk
• Bad news comes out about safety of the product- eg, E Coli found in sprouts
• The price of the products goes up sharply- eg gasolene
• Special taxes are added to the product- eg tobacco