[dageshi]

Typically, it's because most of these businesses have some extra requirement. They need a brochure site + some complicated multi-step form, or some integration with a third party API. Wordpress is popular because if there is any platform for which there will be a turnkey plugin solution for these extra bits of functionality, it'll be wordpress.

The API will have an official plugin, or someone will have written one because their client needed it and the dev decided to support and sell it. Complicated forms can be turned out with plugins like Gravity forms.

This is why lots and lots of sites are still built on wordpress because the ecosystem around it lets people do a lot of stuff quickly without touching a line of code.

[joe_momma]

I could see the benefit of creating many forms or ones that frequently updated.