I suspect another big reason is, if you're mostly a backend guy and you're building the site and can't afford someone to worry about the front end, how do you m

I suspect another big reason is, if you're mostly a backend guy and you're building the site and can't afford someone to worry about the front end, how do you make your site responsive and mobile friendly? Bootstrap. It's (extremely) easy to learn and (as mentioned in other comments) its ubiquity means people come to expect it and are already familiar with the UI.