It takes time to prepare such announcements, not only to write...

It takes time to prepare such announcements, not only to write and word them very carefully, but to pass through the gates of management and create certainty about what is bein said. The end result can sometimes be a long time after the initial reason for the trading halt, which can be as simple as someone in the lab saying "eureka!". As a publically traded company, they have a duty to share holders to prevent insider trading hence price sensitive events like lab results will produce a halt even if they're not ready to release an announcement yet.