My speculation:
1. They recycled the front panel design from other instruments (e.g. function generators). If they wanted space for three binding posts, they would have had to redesign the front panel.
2. Because these are precision power supplies, substantially more expensive than standard bench supplies like the E3617A, I suspect a large fraction of users would use the connector on the back so they can use remote sensing. Compared to a more basic power supply that is more likely to be used without remote sensing.