Possibly because he knows JS+CSS, but not C, C++ or C#.
Or possibly because the API's microsoft proposes are pretty horrible; I've worked on FF and chrome extensions, and I investigated IE support - but IE's extension model is truly terrible. Just take a look at that stackoverflow link you posted. What a mess. I wouldn't mind a clean C++ or .NET api, but the current API certainly isn't that.
Because the latter is a lot quicker to get started with. I'm not making a value judgement.
With the IE model, you need to create a VS project, add a ton of boilerplate (see that stackoverflow example), and generally it's a ton of work just to get started. With a Chrome extension, you get an itch (some website has some jank, or you just want to hide all images with a certain dimension), you peek at it with the inspector, write some lines of JS or a custom CSS file, add an XML file and you're good to go.
Being able to write a quick hack to scratch an itch makes the Chrome/FF/Safari model more approachable. Once you have your foot in the door you're then more likely to make the step to write a full-featured extension (your MVP gave you a bunch of ideas).
It's similar to why PHP is popular. "I have this HTML page and I just want to add this one dynamic visitor counter. Do I want to build a Rails application, or paste in 3 lines of PHP and change the filename extension?"
Or possibly because the API's microsoft proposes are pretty horrible; I've worked on FF and chrome extensions, and I investigated IE support - but IE's extension model is truly terrible. Just take a look at that stackoverflow link you posted. What a mess. I wouldn't mind a clean C++ or .NET api, but the current API certainly isn't that.