Only Eastern Supreme Kai is left, why he does all the job? There's supposed to be 4 Supreme Kais and 1 Grand Kai
We know the answer to this in the Majin Buu Saga in Dragon Ball Z. While Shin lived in the sacred world of the Kai's, Bibidi(Note: Babidi's father), targetted them and wanted to kill them. So he unleashed Majin Buu(Note: Kid Buu, his actual form) on them who ended up killing the North and West Supreme Kai. Kid Buu apparently then absorbed the Southern Supreme Kai and ended up transforming into Super Buu.
Finally, whle this monster went after Shin and the Grand Supreme Kai. The Grand Supreme Kai intervened and was absorbed, which turned Buu into the Fat Majin Buu(The playful Buu we see the first time Babidi summons Buu). Hence he spared Shin's life. Shin later caught Bibidi off guard and destroyed him while Buu was in a ball and later hid the ball under the surface of the Earth. Since all the 4 supreme Kai's had died, Shin became known as simply the Supreme Kai who lived along with his body guard Kibito in the world of the Kai's.
This clip shows the scene where Shin narrates the same.
With regard to why new Supreme Kai's weren't picked, there isn't a definite explanation as to why. I believe this theory of multiple Supreme Kai's has been retconned for the sake of simplicity. Here are a few reasons why
- In the Universe Survival Arc, we see only 1 Supreme Kai from every Universe and they are referred to as "Supreme Kai" and not "Grand Supreme Kai" .
- The first reason indeed isn't completely factual but if you look at the episode where Goku, Beerus and Whis go to the Sacred World of the Kai's in Universe 10, we see Gowasu with only his apprentice Zamasu. We don't see any other Supreme Kais. Also, it would make sense for Gowasu to train Zamasu to make him a Grand Supreme Kai. He was training to become a Supreme Kai. So if other Supreme Kais exist, they obviously could've trained him instead.
- Shin is considered to be the weakest and least experienced Supreme Kai compared to the Supreme Kais across the multiverse. It wouldn't make sense for them to recruit a few more Supreme Kais(More inexperienced than him, because if they were more experienced, they would end up becoming the Grand Supreme Kai.
- Going back to the Future Trunks Arc. Zamasu only tried to kill Gowasu(In the present timeline). There is no mention of him trying to kill other Supreme Kai's in the same universe.
- Finally the best reason why I think this theory has been completely retconned is based on Episode 55. In this Episode, Whis states that The "Kaioshin and Hakaishin(In Singular) are a set" when Goku asks was curious at to Beerus was concerned about Shin
Of course, this argument is indeed debatable and the only one who could give a definite answer would be the writers themselves. However, based on the information we have. I think it is fair to assume that this theory has been retconned for simplicity reasons.
A possibility, though I don't think there's anything canon that specifically says this:
The quadrant based Supreme Kais were part of a system specific to Universe 7, arising out of unique and remarkable circumstances.
There are a couple of things (from Super alone, no less) that tell us there can be multiple Supreme Kais at once:
Gowasu's "temporary promotion" of Zamasu to Supreme Kai rank, while Gowasu retained all the privileges and abilities of that rank concurrently.
From Universe 7 we have Shin and the Elder Supreme Kai that was sealed in the Z Sword.
So there can be multiple entities holding the title of Supreme Kai at the same time (indeed, the number of concurrent holders seems only limited by the number of potara earrings available, as wearing one was necessary for Zamasu's temporary promotion).
The reasons there usually aren't several at once is because Kais aren't born very often; they live for extremely long periods of time; a single worthy kai is usually sufficient; and kais only rarely show the capacities to be a Supreme Kai.
Universe 7 may have just had a lucky glut of 5 kais that were deemed worthy of the title, so the "Grand Supreme Kai" plus additional quadrant Supreme Kais system was developed to account for this. The Grand Supreme Kai in this case may have been the singular Supreme Kai from before this situation arose, and the quadrant kais would have been his disciples.
In the fourth volume of the Dragon Ball Super manga, there is a gag panel where Gowasu asks Shin why the Grand Supreme Kai didn't tell him about the Potara time limits. Shin suggests he didn't fuse to stop Majin Buu because he probably didn't know.
But I have not been able to find this panel to verify. If you believe this statement, and are willing to take a (presumably 4-koma style) "joke" panel as nevertheless indicative of (manga) canon events, then this would confirm that the Grand Supreme Kai was not retconned, and indeed that other universes' Supreme Kais, like Gowasu, were aware of it.