It's the same reason why any old fad isn't in vogue anymore - it's just how popular things go. I mean, message queues weren't exactly new things in the late 2000s... your standard GUI and mouse uses message queues, pretty much since the 1980s. More and more people just caught on over time, popularity hit a peak, and then people eventually moved on. They're still used in many places, just no longer what's being crazed about.