If you are overly dependent on any single place or traffic source – it could be very disastrous if that traffic source went down. For example, if I was only dependent on YouTube traffic and YouTube had problems (if it went down, ceased to exist, etc), it would be disastrous for my online business.
Sometimes the SEO rankings on search engines are constantly changing which could be a problem if you’re relying on one source. It’s better to diversify your traffic sources to keep your revenue stream steady in case of a dip in one of your sources. If you have lots of other assets and ways for potential buyers and customers than if one source completely dies off, you’ll still be okay, because you’ve minimized your risk by diversifying.
Things don’t stay static, in life, history or business. It’s always a good plan to diversify and keep an open mind when acquiring your customers.