Well, this probably isn't the only reason why IIS has such poor market share, but it is certainly the reason why I won't be using IIS for my work.
My computer at work was recently reformatted using a "Windows XP Pro SP2" installation CD. After the install, I updated (on recommendation from Microsoft's Windows Updater) to SP3. I did this immediately because of security concerns.
When I tried to install IIS to do some testing of websites and services that I've been developing, I was told that I needed to insert my "Windows XP Pro SP3" CD instead. I don't have an SP3 CD, just an SP2 one. Too bad, can't use that!
So I do a Google search for "IIS", and then do a search for "Download IIS". Most of the results that pop up are forum and mailinglist posts from people having the same kind of problem that I am having. So I went to iis.net, a website that, I'm positive, must have a download link somewhere. So I click "Downloads", and can't find it. I click the link "Try IIS7", but that isn't it either. I search through the list of downloads for about 30 minutes before giving up in despair. If there is a download link to get IIS from this website, I can't find it. Well, I could find a trial download of Windows Server 2008 that came with IIS, but I'm not reformatting my computer again to install a trial version of the OS just so I can have the privilege of using IIS (and then being nagged forever about how I'm using a trial version and I need to purchase a legitimate license). There are some forms of obnoxiousness that I absolutely refuse to tolerate coming from my computer.
So I click on the "Chat live with a specialist" link, which insists that it's going to be a "live" person. A window opens up that says "Liveperson", and shows a picture of a live human female. I spend the next 10 minutes chatting with a bot. Actually, it could have been a person who was so heavily scripted and ignorant of their work that they failed the turing test completely. Here's an actual quote, to get a sense of what I mean, keep in mind that we are on Microsoft's iis.net website where every single page literally has a reference to ISS on it: "I see. To make sure we are on the same page, can you tell me what IIS is an acronym for?". "She" tells me that she can't be of any help (probably because "she" is just a bot) and gives me a phone number to call to get in touch with the sales manager. No thanks, I don't want to buy anything, especially when it's this much of a hassle.
So I'm not going to use IIS on my computer. I've given up on it. It's obviously too much of a hassle, and the benefit is only that I'll be able to develop and properly test IIS-based websites locally. When we start designing our next generation product line and developing our next generation platform, I'm going to suggest to my boss that we use a web server that we will be able to get easily and use for proper testing: Apache.