Because many people who design webpages aren’t very great when it comes to typography?

It’s just a bad idea to use those light fonts for anything but headlines or other kinds of large type. For the headline that light font is absolutely ok. For the body text it’s just embarrassing (especially since this is from Adobe).

I have to say, though, this blog is written by those at Adobe who develop new standards (for HTML/CSS/JS), so probably very technical people who are more often than not not very knowledgeable about design – that is to say, those involved here do not necessarily have a strong design focus (and neither do they necessarily have to have that).

Maybe the one who decided it has an Apple device with 'retina' screen? I was reading it on an rMBP and the font struck me as very pleasant, hence I couldn't understand the fuss here first. Helvetica Neue Light is quite popular on these newer Apple devices, it's good to know that it is a bad idea though.

I have been told that it looks "expensive".