Because then you have to slow down to keep a safe following distance again. Then it happens again, and again, and again.
1. It's the fault of the driver behind, for not leaving space
2. It's impossible to leave space, because cars will move in if there is a large gap
3. Why is this a problem?
"To keep a smooth traffic flow, some highways also have minimum speed limits. If you drive slower than the minimum speed you can halt the traffic flow and create a dangerous condition. Even if there is no minimum speed limit, those driving too slow can be as dangerous as those who drive too fast."
https://dmv.ny.gov/about-dmv/chapter-8-defensive-driving#yre...