The keyboard shortcut for putting focus on the address bar in Explorer and Internet Explorer (and Edge and the version of Internet Explorer that came with Windows 8 that nobody talks about even though I really liked it) is Alt+D.
Why the letter D?
Okay, the proximate reason is that there used to be the word "Address" next to the address bar, so you knew what that thing was. And the keyboard shortcut for that was "Address", or Alt+D. You can still see the accelerator highlighted on the right hand side of this ancient screen shot.
Over time, the label "Address" disappeared, but the accelerator remained as an accommodation for years of muscle memory.
Okay, but why was it "Address" instead of "Address"?
Because the accelerator Alt+A was already taken by the "Favorites" menu.
But why was it "Favorites" instead of "Favorites"?
Because the accelerator Alt+F was already taken by the "File" menu.
It's a really nerdy chain reaction.