Easy, it's a POSIX-compliant OS and the vast majority of the web is deployed on servers with a similar environment. This is doubly true of the consumer-oriented web apps that Marco's talking about.
I'd disagree that this has any relation to it being 'better for developing web apps'. I write code and work with teams coding a multitude of languages and environments (on Windows, Mac, and various nix flavors), and the developer experience in the Microsoft toolset is pretty impressive.
I'd expect the shift to be more a function of cost, since it's a lot easier for a startup to spin up on OSS vs. procure Windows licensing, and it's also a lot easier to get free hosting deals like Heroku, etc. than Windows (although I expect BizSpark is changing some of that).
So I do agree that there are some appealing reasons to start projects on nix and OSS, but ease of development and quality of tooling (i.e. a 'better development experience') is likely not at the top of the list.
I'd disagree that this has any relation to it being 'better for developing web apps'.
As someone who dealt with the nightmare of trying to develop Ruby on Rails apps on Windows in 2006, I assure you that this was a significant issue.
I'd expect the shift to be more a function of cost, since it's a lot easier for a startup to spin up on OSS vs. procure Windows licensing, and it's also a lot easier to get free hosting deals like Heroku ...
This is also true.
In general, Windows was not an ideal environment for developing for the web.
Oh, no debate on trying to build Ruby apps on Windows (been there, done that - which is why I have a separate Ubuntu partition for that). But more the point that if I am building a web app in, say, ASP.Net MVC using Visual Studio 2010/2012, the tooling is excellent.
Since the article is talking about Windows Developers, it's probably fair to assume that most of them are knocking out code in Visual Studio. And regardless of one's opinions on Microsoft, it's hard to argue with the quality of their developer tools.
I'd agree then that Windows is not an ideal environment for Ruby development, but not all web dev is in Ruby, and windows is an excellent web development environment if you're building out on the Microsoft stack.
The article is talking about the 2004-5 time period. ASP.NET MVC didn't exist, .NET was still at v1.1, IE was still at v6, Windows was still at XP, etc.
(I'm probably an anomaly in that I actually switched to Windows - from Linux - a few years after that, around 2008)
I see pretty comparable numbers of listings for Python, Ruby/RoR, Java, and .Net (although if you add up all of the nix web dev jobs and bundle Python, PHP, and Ruby together vs. .Net, you're going to see a lot more of the former).
For web dev in general the split is pretty even, but if I am looking more at startups or small shops, the balance shifts towards Python/RoR (with Javascript being pretty ubiquitous whether you are on .Net, Java, RoR, or Python).
Still see this more of a tooling cost issue though, which makes sense. MS licensing is not cheap, although I've worked with a suprising number of startups that are on the MS stack, or using a combination of languages.
Do you think all those ASP.Net sites are more likely to be delivered on the web to a diverse selection of evolving, modern user agents, or on an internal network to only IE7 users until 2019?
One opportunity has a choice in development tools. The other doesn't, really. The former market for developer mindshare can and does change; the latter is Microsoft's entrenched base which is never really in jeopardy (nor very relevant to the growth of the tech field at large).
