Because of its price the 'wasabi' you're used to is probably just a mixture of horseradish, colouring, and sweetener. These products often only have 1% to 5% of the real thing in.
Wasabi is known for being the hardest plant to grow commercially in the world. It can be found naturally growing alongside Japanese mountain streams has a strict set of conditions it needs to thrive.
Wasabi needs a constant supply of running spring water; it likes a shady area and rocky soil or gravel; and can only tolerate a temperature of around 8 to 20 degrees centigrade all year round. Too much humidity, or the wrong minerals can also cause problems for the plant and on top of all that it's susceptible to pests and disease.
There's one other reason you probably don't see real wasabi products in your local supermarket or restaurant. Wasabi's spice comes from a chemical reaction that occurs when you break down the cells, but this reaction is short lived. After 5 minutes the spicy flavour peaks but leave it for 30 minutes and almost all the flavour is gone.
All of these factors mean fake wasabi isn't going away any time soon.
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