Digging a trench isn't the same. You're not cataloging and gps'ing everything you find, photographing it, and marking it. When you dig a trench, you're digging to find that line, which is already marked and gps'd so you just dig to the level you need, then clear it out to the line. This is tedious work, it's done very carefully and depending what they find, it can take time. They'll typically do a surface survey as it's not unusual for scavengers to carry things away from the original site, and they'll do a complete survey of the ground area to gather evidence on the surface. Everything found is marked, photographed, gps'd and then catalogued. After that, they're bagged and taken into evidence. That's all before the digging can begin because once they dig, the area is is destroyed. They excavate only inches at a time, they sift everything, document everything that's found, mark it, photograph it, and then enter it into evidence. Grids are put over the area to mark it, they have to be careful not to dig down to the body so they don't damage it. It's more involved than digging for a pipe under the ground.
They'll also want to notify the family and make preparations before they notify the public.