Because they have a very limited area where they excel. On the other hand, you can do almost anything you like in functional programming. Consider, for example UI. Take a look at UI code in visual prolog: <a href="http:&#x2F;&#x2F;www.visual-prolog.com&#x2F;video&#x2F;FormDemo&#x2F;default.htm" rel="nofollow">http:&#x2F;&#x2F;www.visual-prolog.com&#x2F;video&#x2F;FormDemo&#x2F;default.htm</a>