Honestly. I don't know. Maybe,... because they don't know gulp?
To be honest I started with grunt, too. Beacause some boilerplates used it, and it was the first build system I heard of. But gulp has a nicer and easier syntax and is way faster.
You have to install gulp globally npm i -g gulp
Then create your gulpfile.js and define your tasks.
If you are new to this, I can recommend you laravels elixir
It's kind of a wrapper for gulp tasks and simplify it even more. You can install it via npm i laravel-elixir
then just require it in your gulpfile and define your tasks
I still can't figure out how to set-up Gulp :( I have NPM,Node,Bower installed. I had Gulp installed once I think? I deleted the folder though and yeah. I just can't figure it out. I am only a few months into my webdev path though. I have Koala to minify but I still can figure out SASS and how to make Gulp automate.
Gulp is faster, cleaner, easier to maintain, plugins are optional. Besides Grunt is barely maintained. Its last stable commit on gihub is from 6 months ago, and a lot of grunt plugins are totally outdated.
Personally I do not use neither of them anymore. I've found npm scripts and webpack a better combination for almost every use case I used gulp.