Because they are a waste of money and don't keep the weight staying for very long.
题意:为什么流行的节食法不是很有效?
根据文章第五段:But they'll be wasting their money on this way to lose weight. Fad diets don't work. Or if you do lose weight, it doesn't stay for very long.(但是他们用这种方式减肥是在浪费钱。 流行的节食法不起作用。 或者即使你真的减肥了,它也不会持续很长时间。 )可知本题答案为:Because they are a waste of money and don't keep the weight staying for very long.