Some of it is a preference for clean, semantic naming. Classes like “col-md-1” in Bootstrap drive me nuts, because I can’t remember if it’s “col-med-1”, “med-1-cols”, or “col-m-1.” It’s part of the disgusting programming paradigm where we trade being readable, memorable, and pronounceable for a couple less keystrokes (as if typing were the hard part of development). In Foundation, that same grid indicator would be “medium-1 columns”- much better.
From a more technical standpoint, I prefer SCSS to LESS, think ZF5’s components are more useful and easier to customize, the documentation is better, and things I make in ZF5 look like my work, not just another Bootstrap site.
I heard it said somewhere that Bootstrap is for programmers who don’t want their work to be ugly, and Foundation is for designers who don’t want to reinvent the wheel. I’m trying to move from the first group to the second group, so that has something to do with it.
Great stuff gets made with Bootstrap all the time, so I don’t think it’s a bad framework- if you’re comfortable and productive in it, you should keep using it. It was immediately clear that Bootstrap wasn’t for me, whereas ZF felt like putting on a comfortable pair of jeans from hour 1. YMMV.