I'd imagine it comes down to accessibility. Many people probably have spare Raspberry Pis lying around as opposed to an Arduino with a wifi shield. Arduino would end up costing more and the processing capabilities isn't as powerful as the Pi. Not that the processing is important, but what if you wanted to tack on more functionality like adding a web cam, etc.
The other part probably has to do with language. Being that it's running on linux, you can program with a variety of languages versus just arduino language.
This is very true - at this point what self-respecting hacker doesn't have a Pi or two lying around to be re-purposed?
That said one of my favorite side projects last year was eliminating all of the Pis I had doing stuff like this. I used a NodeMCU ($9 Amazon Prime) and the Arduino SDK to replace my Raspberry Pi garage door opener (integrated with Home Assistant). Complete with magnetic door switch and no prior experience, I'd replaced the Pi after about two hours of hacking. Not worrying about `apt-get upgrade` is nice but the real bonus was noticing how much more robust the esp8266 wifi was compared to the Pi. In my garage door use case the difference was night and day.
Between this example and others around the house, I've noticed the Arduino ecosystem and community is actually much better for this kind of stuff. Once you really dig into things like GPIO, PWM, etc on the Pi it's clear they're second class citizens to community efforts surrounding video/HDMI (as one example). It's actually pretty incredible the same $35 board can control a stepper motor for your blinds or decode H.264 at 1080p while running Kodi in your living room!
My goal for 2017 is to start doing this kind of random hacking - I've slowly completed most of the guides in the Arduino starter-kit, and I've a bunch of esp8266 (D1 minis) on-order from China.
So far the process has been very refreshing, and as a long-time sysadmin working with these parts is a lot less frustrating than having to deal with distros on a pi.
The other part probably has to do with language. Being that it's running on linux, you can program with a variety of languages versus just arduino language.