That's probably because unbiased information about HTML/CSS frameworks would tell you that if you knew the first blasted thing about HTML or CSS -- specifically semantics and separation of concerns -- you'd realize that such 'frameworks' like Bootcrap are nube predating ignorant bloated nonsense!
You end up with non-semantic markup, endless pointless DIV for nothing, endless pointless PRESENTATIONAL classes slopped all over the place so bad you might as well go back to writing HTML 3.2 and pretending CSS didn't even exist, learn layout 'concepts' that really have little business on websites in the first place if you actually care about accessibility...
... and to be BRUTALLY frank, I've yet to see one that actually lives up to the wild nonsensical claim of their being any 'easier' or 'simpler' -- much less the utter rubbish of it making you more productive.
As a rule of thumb, they make you write two to five times the HTML needed, as much of your own CSS as you'd probably have without the framework if you just knew CSS, end up a specificity nightmare that makes maintenance an ungodly headache where you spend hours pouring over the document inspector trying to track down what's applying that rule you can't seem to override...
I'll never understand how anyone who grasps the concept of what HTML is FOR (saying what things are, NOT what they look like) and what CSS is for (customizing appearance for different media targets) could by choice choose a so called HTML/CSS framework like bootcrap, yui, etc. I've NEVER seen a site built with them that couldn't have been less code, simpler code, with 100% the functionality and 90% the artsy stuff WITHOUT the framework...
How writing more code that relies on more code that makes more code that's harder to maintain somehow magically becomes "eaiser"?!? I'll never understand that. Though to be fair I say the same thing about the <style> tag and 98%+ of the time people use style="".
Particularly when they go and piss all over the markup with that OOCSS style asshattery such as:
<nav id="mainNav" class="navbar navbar-default navbar-fixed-top">
Needless to say, my opinion on the whole subject is far from popular; harsh light of truth is often that way: