Why is it still recommended to use tables for email structure?

The main reason tables are still used nowadays is to support Outlook 2007/2010/2013. Those versions use Microsoft Word engine to render HTML, and it's quite a mess. It has a limited support for CSS (no float or position for example), and some CSS properties are only supported on some specific HTML elements. For example, padding is supported on a <td>, but not on a <div>. And even when you could theorically use padding on more semantical elements (like <h1> tags), or use margin on <div> elements instead, Word's rendering engine is still massively bugged and can have unpredictable behavior with such HTML and CSS code. Thus, developers find it easier to just use <table> instead. You can read more about Outlook HTML and CSS support here.

But here's the thing : if you don't feel like you need to support Outlook 2007/2010/2013, then you can absolutely ditch tables and use better code instead. And even if you need to support it, simple one-column layouts can be done without tables. The reason your template works in Outlook 2011 is that this version (for Mac only) uses WebKit rendering engine (just like in Safari or Apple Mail).

I've been doing some work recently to see how email can be built in a less table-heavy way, and weirdly I started to see some odd things happen in some of the "good" email clients when you send an email built in divs. Long and short of it is, as cumbersome as they are, tables are still the safest way to maintain a consistent control over layout.

I have only found <div>'s useful in maintaining order with responsive design and the display work around with Gmail...