Because Frontpage sucks maybe?
Its ugly, produces heavy bloated code. Can cause problems on your server. It has no idea about SEO, has no idea about security. Need I go on?
The sites are no where near as powerful as you can build in a good CMS.
e.g. I put
this together for someone in about 20 hours with Joomla. To do that in Frontpage would take weeks if not longer.