Just a wild shot in the dark here, but could it possibly be because of an increasing number of people have access to the internet?
"Programmers on the internet" has reached saturation, but the total number of people searching for things on google is increasing exponentially?
The chart doesn't compute the ratio of searches (for those keywords) against the total volume of Google searches; it shows the absolute query volume for each keyword. In other words, it's independent on the number of people not searching for those terms.
Are all those languages' searches really going down since 2004?
Yeah. It is hard to find a language that hasn't been going down since 2004.