@Prince-kun
Eh, they actually touched on that.
It's definitely not his family, he's confirmed as being from a Standard Low-to-Middle-Class family.
It's pretty much the Core Elders responsible for starting things. Mu Suyin signed up on a whim, but when they looked at his background, he basically was one of the top political science students at Daxia University (Professor Wang admits he was a good kid and never lost any debates in his class, even if he said his actions were basically too naive and not realpolitik enough after becoming an Admiral) and had spent at least a bit of time trying to help the less fortunate out (he was an advocate for a bunch of poor indentured workers and tried to get them their pay or at least passage home) only to fail because he didn't have any power to make people listen to him.
So, the Core Elders took a look at his application, found an idealistic young man who was attempting to live up to principles, then figured 'hell, let's give him some actual power, Daxia is getting way too stagnant'. I thiiiiink that's why Ding Yuan and Himiko both showed up to recruit him when he sent his application off. Both of those fleets needed new Admirals at the time (Dragon Slayer Alliance has apparently been planning their shenanigans for at least two decades at this point, so there's a reason the Daxia Fleets were running through Admirals quite quickly. Kunlun avoided that because, well, good luck killing a Heaven-tier Angel with the strongest defensive materials), so the Elders sent it to both of them.
Keep in mind, the major Daxia fleets tend to be noted to be 'peasant' fleets, so they don't tend to have a lot of members with higher educations. The rank-and-file Angels tend to join up because it's 'risk life as Angel or starve' and Beiyang's fleet in particular seems to have an infestation of Idle Rich 2nd Genners coasting off ancestral achievements and massive corruption among a number of Planetary Lords. Three-Legged Crow has less of the innate corruption so far, but was smashed hard enough after the God-Slaying War that Ditu apparently basically put them firmly under their thumb which is pretty troublesome. Kunlun's the strongest of the three, but also seems to be mostly defensively oriented, so it seems to do less conquering and resource acquisition than the other two major fleets.
Mind you, Mu Suyin avoiding assassination via Holy Blood by having a strong enough will to overpower the ancient Elder Race Dragon at the bottom of the pool trying to body snatching and body-snatching them instead, allowing him to break through to Soul-Tier, wasn't really in anyone's calculations, but the Core Elders are rolling with it and taking full advantage of him both cleaning house and being a massive obstacle to Dragon Slayer Alliance.
It lets them catch up on their Mahjong!