Because we are very curious and like to know everything...
Seriously, it already happened that an a/c requests diversion because of a tech problem, but doesn't want to say so, so that he is not charged with all the assistance stuff on the ground.
Then we like to know what's going on, if we can expect something that goes wrong, and maybe we advise the emergency services.
We also are going to be asked the reason why, so better ask first..