There are a number of reasons why this might be the case. There are two main things that a rocket has to do. The first is to get them to the right orbit, and the second is to make sure the payloads get deployed safely. The reasons why such a long wait might include:
- The satellite needed to be deployed over a specific area to ensure it received sunlight when it was deployed.
- The primary payload might have wanted to make sure it could deploy its antennas and solar panels and even execute a small burn to get it further from the rocket before secondary deployment.
- They might want the deployment to happen at a location where the rocket is in communication with the ground.
And there are likely others. The bottom line, if the deployment happens after a long coast, it must be for the benefit of the spacecraft to do so.