Could be for example some tape robot where you can have huge amounts of tapes in the storage but only have few devices for reading/writing them. With tape you can't really stream the data to the web. Instead you would probably first copy it somewhere. If there is lots of data, say few terabytes, even this process takes some time.

Or in case they are using regular hard drives, you might want to have this kind of time limits in order to pool requests going to a specific set of drives. This would enable them to power down the drives for longer periods of time.

The 3-4 hour estimate may also be artificial. Even if you can in most cases retrieve the data faster, it would be good to give an estimate you can always meet. They might also want to differentiate this more clearly from standard S3.

And we should not forget that it does take time to transfer for example one terabyte of data over network.