Why is so little known about how xiangqi actually spread and developed?
These answers restate the rules implemented by our open-source engine, which follows the WXF laws and is regression-tested against the official 110-diagram repetition casebook. Where an answer refers to a specific position, that position lives on the linked reference page, where the build verifies it against the engine before publishing.
Direct answer
Because for most of its early development, xiangqi's rules were transmitted the way a living game usually is, through play and teaching, not through formal documentation. A rule change that succeeds simply becomes how the game is played; nobody necessarily writes down that anything changed, or when. Written manuals only start appearing once the game had already largely settled.
Why this is normal, not unusual
The same is true of most traditional games in most cultures. Chess's own early medieval development is patchy for similar reasons, well before the printed opening theory that later generations take for granted. Documentation tends to arrive once a game is already mature enough to be worth writing about at length, which is naturally after most of the formative changes have already happened.
What we are left with
A reasonably solid picture of the game from the Song dynasty forward, once literature and surviving manuals exist, and a much hazier picture of exactly how it got there. See the classic manuals for what actually can be traced in the better-documented later period.
Related questions
Does this uncertainty apply to other old games too?
Yes, it is the norm rather than the exception for games that predate systematic written record-keeping.
Could new historical research change the picture?
It is possible, though any claim about the pre-Song period should be treated with real caution until it is well supported.
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