Oh, there are logical reasons for that:
1. Pernik and Sofia are much closer together in real life than the game suggests. Any road going from Pernik to Plovdiv either takes a long detour south, or ends up on an existing road in Sofia, for which there is no space on the map to add another road.
2. IIRC, The E87 border crossing has a truck ban, which is why SCS didn't make it. I think it's because Turkey wants extensive truck checks on the border and the M. Tarnovo border crossing isn't up to standards.