Hi masfenix, welcome to TalkPHP
I'm a .Net developer in my day job (C++/C#, mostly Winform stuff, but occasionally some ASP) so can understand where you're coming from.
I believe that the sucess of PHP is based on 4 major factors:
(I should point out that these 4 points are based on 5 years ago with the original ASP, not ASP.Net)
- Open Source - people do love their free and open source programs
This certainly helped with early PHP adoption. Until recent years (VS Express), you had to pay good money (and for some, a lot of money) for Visual Studio if you wanted a full-featured IDE for ASP (as far as I know, correct me if I'm mistaken and MS had a free ASP IDE availavle).
- Linux Hosting - traditionally, Linux shared hosting has been cheaper than Windows hosting. Whilst this isn't the case nowadays, 5 years ago, Windows hosting would cost more due to the Windows Server licencing costs. This closely ties in with point 1, people do love to save money
There where various attempts to get ASP code running on Linux but none of them where particularly great.
- Anti-Microsoft - As sjaq and Orc mentioned above, they don't care for Microsoft as a company and Microsoft products. Unfortunately, this view is shared by a lot of other people which has helped push PHP.
- Beginner Friendly - PHP has always been thought of as having a low entry requirement for beginners (which in some cases, turned out to be a bad thing), where as ASP always had the stigma that it was hard to learn.
All these points helped PHP's early sucess as a web scripting language, particularly in recent years as the Open Source "movement" took off.
Saying that, PHP isn't quite the king of web scripting languages that a lot of people think it is

ASP.Net is the language of choice in the corporate market. In my experiance, corporates know and trust Microsoft as they run their business on the Microsoft platform so they naturally choose ASP.Net over PHP/Ruby/JSP/etc when moving to the web.
As for my own personal opinion, I use ASP.Net in my day job (not a huge amount I must admit, but occasionally we get to play with it), and PHP as a "hobby" (programming as a hobby - I need to get out more

) so I can't really say that I favour one over the other. At the end of the day, I'll use whatever language is right for the job at hand - anything to make my life easier
Alan