With the last chapter, we now know why we had never heard of Issho prior to chapter 701. Everyone was wondering where Vice Admiral Issho came from and how he became Admiral Fujitora. We now know the answer, this is his first Marine job, same with Admiral Ryokugyu. Sometime while the main characters were away, the Marines held a global military draft/enlistment (not sure on the translation). It would seem that one of a few things would prompt the Marines to do the real world equivalent of drafting people into the ROTC (Reserve Officer Training Corps).
The first possibility is that they are short staffed. We know Sengoku, Aokiji, and Garp resigned, creating a hole in the ranks, the first of which was filled by Sakazuki, creating another hole to be filled by Issho and Ryokugyu, and lastly Garp's by Smoker and/or Maynard. This gave everyone the impression that someone from the Vice Admiral's Club got promoted to admiral, keeping the number at 16. Then this draft comes along and suddenly we now have 17, which irks me. Some time later, Ryokugyu's name gets dropped by Doflamingo. When this happened I was glad to learn the name of the third admiral, then my spirits were dashed as I finished reading that sentence, where he mentions this Global Military Draft. The good news is we know for sure that we don't know the new admiral. In short, all bets are off. You would think 16 would be enough vice admirals, but no. Interestingly enough, more known characters hold the rank of vice admiral than any other rank. Although it is possible that one of the current vice admirals left the Marines. If I had to guess, I'd say Tsuru, just because of how unenthused she seemed at Marineford. But back to my initial point, the bigwigs decided to hire outside instead of promoting, which suggests that all commodores, rear admirals, and vice admirals have more than enough on their plate to handle. And they should. Shit went crazy after the war, and the Marines' resources are probably stretched pretty well. So, when no one from the inside can do it, they placed a classified ad. I imagine it went something like this:
Admiralty! We need to fill two spots and we're looking for the right people to do it. Do you think you have what it takes to fight with the big dogs? If you do, then pick up a touring schedule to find out when the judges will be on an island near you! You've seen them at Marineford now come see them live as they decide whether or not you have what it takes to be an admiral. Become a contestant on
I Want To Be an Admiral!
Now imagine something like the show Shark Tank only instead of Mark Cuban and three other rich people it's Kizaru, Sakazuki, Momonga, and Tsuru.
There is also the possibility that no one wanted the job for whatever reason. And who can blame them? How often do admirals get dispatched on assignments? Admiral Kizaru made his debut only because Luffy punched a fat rich guy. Akainu was sent out to the burning tiki island when the BB pirates asked for a ship. And now Fujotora was dispatched to Dressrosa because of Luffy and Law. That's less than one time each per admiral during active duty. They might be the strongest fighting force and whatnot, but they sure don't get to do much. The vice admirals that don't have desk jobs at HQ probably like not having desk jobs at HQ since it means they get to see more action.
Another possibility is, and don't shoot me just yet, the bigwigs did not think any of the vice admirals were good enough to become admirals. Now, I know what you're thinking, something like "WTF are you talking about!? Vice Admiral INSERT NAME HERE could wipe the floor with blah blah blah". Am I right? Don't blame me. I'm not the one who decides Marine promotion policy. Sakazuki may have set a threshold that none of the current crop could meet. Or, putting myself in the mind of our good fleet admiral for a moment, maybe he wanted some fresh ideas. The current crop seem pretty set one way or the other, either they follow Absolute Justice completely without question while some are reluctant, or they follow it to less of a degree. So Sakazuki wanted some people who weren't brought up with the Marine mindset. Why did the most devoted follower of Absolute Justice want to do that? I don't know, I just know what he did.
What really surprises me about this draft thing is the intrusiveness of it. The Marines almost never interfere directly with the internal affairs of individual nations. What am I talking about? Let's break it down.
Global: Of or relating to the entire planet.
Military: The thing that protects most nations.
Draft: Required service.
So, basically, the Marines broke protocol and intruded directly on the internal affairs of several if not every country that's part of the World Government. They then decided which high ranking officers they liked, and after some eliminations, chose two to be admirals. Personally, I would have just done it like American Idol, by which I mean you make the talent come to you. If I had to guess, based on his outfit, Issho comes from a winter island. Overall, this can be seen as a sign of weakness and doubt within the organization, which is never good in their line of work. If it were me, I'd have promoted Bastille and Lacroix and just let their size serve as an intimidation factor while Kizaru did all the work. At least that way I could save face with literally the world.