I've been trying to attack this question (or more precisely, come up with an answer to that fact) for some time now, but after a while of research I'm suddenly not so sure of the reason the situation is as it is.
I used to think that nihilism today is so popular because of the uprising of positivism in science and scientism in the past century, but I'm not exactly sure anymore. I know some blame it today for post-modernism, but I'm not sure it's quite correct too. So, I'd like to see what people here think is the core issue that made room for the popularity (of course there are those who will argue with me about its popularity too) of nihilism in the last few decades.
I should say, while it is not directly related, I am going to connect atheism in some way to nihilism, as in my opinion they both came from the same source last century, but I would definitely accept other point of views here.
Edit: while writing a comment to @GeoffreyThomas' answer, I realized the two definitions of nihilism I'm talking about:
Postmodern nihilism - the postmodern position that says "there's no one truth", which leads to a nihilism that denies objectivity, and is more popular today because, well, postmodern is quite popular today as it's a new school in philosophy.
Existential nihilism - the existential crisis of "there's no meaning to life, we are nothing compared to the billions of years of the universe", which leads to a nihilism that denies any sort of hope and aspiration for a meaning.
While I'm more interested in the nihilism that's rooted in the second definition, I'd love to hears answers about them both.