An almost permanent fixture for Australia at loosehead over the last few seasons, Robinson's exit clears the way for either Scott Sio or James Slipper to face New Zealand next weekend in the opening match of the southern hemisphere tournament, which also includes South Africa and Argentina.
Loose forward Dave Dennis was another mildly surprising omission from the squad, although McKenzie was keen not to close the door on any of the players left out.
"It's unfortunate that a number of good players missed out, although this isn't necessarily a case of poor performance but rather a reflection of some of the quality we have coming through in Australian rugby right now," the former Test prop said.
"For the moment, though, we've narrowed our way down to a group of players we believe are capable of getting the job done against the All Blacks in just over a week.
"That doesn't mean the players chosen won't change in the future and that those who have missed out this time will be forgotten.
"I've had a good chat to Benn about what I think," he added.
"He knows what he needs to do. We'll keep him close and he's got every chance of pushing his way back in."
Uncapped Sio was one of 11 ACT Brumbies players included in the squad in a reflection of a fine season for the Super Rugby runners-up.
Sio put together a string of strong performances in the Super Rugby playoffs in a front row with Stephen Moore and Ben Alexander - both likely starters in Sydney next week.
"I've watched him perform over the last three weeks against some pretty tough competitors in some pretty tough environments and he's looked pretty good," McKenzie said.
"He's got a good base for a front-rower. At 21, you've got to say if you put two years of investment in him, how's he going to be at World Cup time? "Those are the sort of things you've got to have at the back of your mind ... sometimes you've got to be proactive and get on with it."
Scrum-half Nic White, fly-half Matt Toomua, centre Tevita Kuridrani and abrasive lock Scott Fardy were the other uncapped Brumbies in the squad.
The final three rookies were number eight Jake Schatz, fly-half Bernard Foley and Albert Anae - a late addition to this week's training camp who can play at hooker or prop.
"I'm quietly confident we'll perform well," McKenzie said.
"We've got a lot of guys that have been out there doing the business in recent times.
"There's a lot of potential. It's a chicken and egg exercise, when do you give a guy a chance? But some of these guys have earned a go.
"We need to find a bit of new territory."
Forwards - Ben Alexander, Albert Anae, Kane Douglas, Saia Faingaa, Scott Fardy, Liam Gill, Michael Hooper, James Horwill (captain), Sekope Kepu, Ben McCalman, Hugh McMeniman, Stephen Moore, Ben Mowen, Jake Schatz, Rob Simmons, Scott Sio, James Slipper.
Backs - Adam Ashley-Cooper, Quade Cooper, Nick Cummins, Israel Folau, Bernard Foley, Will Genia, Tevita Kuridrani, Christian Leali'ifano, Jesse Mogg, James O'Connor, Joe Tomane, Matt Toomua, Nic White.