Haiku Elvis
Knuckle-dusters, glass jaws and wooden hearts.
Why is he choosing a life of danger and hardship over the pampered life of a royal? Well clearly he isn't that smart. 

Their a trading country because they had to, not because they wanted to, at the time the campaign starts the 4 neighboring countries have hated each other for almost 1,340 years, which means the hatred started less then 50 years years after The Pcs Homeland was foundedFirst off, nicely done! The very first PC I ever played, after a similarly low-odds rolling sequence, also turned out to be first in line to a throne. He was a Ranger, and the justification for his field adventuring was pretty easy: he (and his parents) felt he needed to get out and toughen himself up a bit before settling in to the life of a noble.
As for your PC being a Thief, you say the throne he'll one day inherit is that of a trading nation. Well, what better way to learn all about trading - the good side of it and the bad - than by becoming a Thief and learning it in practice rather than just in theory; and what better way to quickly advance your skills and knowledge as a Thief than by a bit of field adventuring.
As for why would the future king have merely average Int and Wis, maybe those traits run in his family. Or maybe the throne comes with some magic items (or, more likely, very competent advisors!) to cover off these shortcomings.
Because his name is Conan.why would A Future King become A Adventurer?, and even if he did why be A Thief?
And with 4 younger siblings it’s not like the kingdom fails if they fall.Being descended from royalty doesn't immediately translate to having a high intellect or great wisdom, nor good moral character (thief). In fact, there have arguably been royals IRL who were stupid, unwise, and/or swindlers. It might conceivably mean they don't need to adventure (as they probably have access to money), but perhaps they merely want to adventure (they may be a thrill seeker).
Charles II had 9 great-great-grandparents, instead of the usual 16.[Edit: For a notoriously dimwitted king, lookup Charles II of Spain. For notoriously immoral royals, look no further than the House of Borgia.]
Or it could be A Female Noble whose A Magic User that wants to marry my characters younger brother and because she has access to The Spell Charm Person, which she could use on my characters younger brother, and if it works my characters younger brother would be the official ruler but in truth his wife would be the rulerShakespeare's Henry V features a low-stat heir to kingship (the Dauphan) and a king who used to run with some rabble (Henry).
If your character has an ambitious younger brother who wants that crown for himself, he might have engineered your exile from Court - or faked your death - and you are trying to adapt to unexpected circumstances. (Also works if ambitious high-rank noble has eyes on using your sister as his path to the crown).