Tight coupling and money are a good arguments, thanks. However, i believe we are comparing apples to oranges here. IIS uses it's own kernel module, HTTP.SYS - http://www.microsoft.com/technet/prodtechnol/WindowsServer20... So, looks like having specific kernel api optimized for your particular usecase is an advantage, which IIS has and others don't.

It turns out there are kernel-mode webservers (or there were, at least), which seriously outperform user-mode ones. https://www.usenix.org/legacy/events/usenix01/full_papers/jo... (page 11; article is old, but anyway)