On a related note, why would it be so devastating to Trill society if it became known that they have a lot more hosts physically capable of hosting symbionts than they have symbionts? They’re a reasonably mature and democratic society, aren’t they?
Even mature, democratic societies will often evolve to include political elites that end up dominating that society to the exclusion of people in the lower classes.
The political elites of Trill claim that they've been perpetuating the myth that only a small percentage of the population can host a symbiont because they were worried that if it was more widely known, lots of greedy people out there would reduce the symbionts to mere commodities, to be bought and sold at will.
On the other hand... Look at it from the point of view of an Unjoined. To a member of the Great Unjoined, it could easily appear that the Joined are actively seeking to undermine the ability of most Unjoined to move up in society. Being Joined tends to give you great advantages in Trill society -- Trill: Unjoined
establishes that the Joined make up the vast majority of the Trill government. The Trill Senate is mostly Joined, the Trill President is usually Joined, etc.
There's a strong case to be made that the Joined are actually only seeking to protect their privileged status within society by keeping secret the fact that most Unjoined could carry a symbiont successfully. And when people feel like political elites are keeping them down -- are actively oppressing
them? The results are rarely pretty.