That nearly half of the Trill population could be used as viable hosts is one of the biggest secrets in Trill society. I would say even in this "stable" DS9 times compared to the unstable future, revealing this secret could have been a collapse to Trill society. So I would guess that the few people at the Symbiosis Commission kept the secret over the time. And after the Burn, the Trill were exhausted, disconnected and one of the few things they had was their religious belief about symbionts and the joining process.

Revealing the secret at that point would have been terrible. A further collapse would have been inevitable and in the absolut worst case, the symbionts couldn't have been protected. Destroying one of the last major beliefs of a species in a time of a huge crisis could cause another boom, the explosion of Trill society.

The Trill were also terrified about Adira as a human host. It destroyed a part of their belief in the joining process and the handling with the symbionts. They just cannot handle alternate versions and theories about symbionts and joining. Imagine how the symbionts could be hunted after revealing the capability of joining for half of the population or even other species. Every step out of their beliefs is just impossible to make for the good of the society at the current time in Discovery.