<main> came late - the others were developed while the HTML5 process was being actively watched and talked about (before people got bored following it) so noone

<main> came late - the others were developed while the HTML5 process was being actively watched and talked about (before people got bored following it) so noone really reported widely on the creation of <main> and as such it's rarer. Hence <div class="main"> <section>, <header> and <footer> are not too uncommon. <article> is by far the most used, and is also put to genuine use in parsing by things such as Firefox/Safari's Reader views and other such tools. <section> gets used similarly by some parsers I think, though I've no direct examples off the top of my head.