Because the design of the road (which has evolved over time, and not always in a thoughtful way) makes it comfortable to travel at 100 km/h, and everyone does that or more. And the traffic volume is high.
It probably should be a dual carriageway with a 90+ speed limit (think 35/115), and it probably should reroute around the villages instead of through them, and at least some of the junctions should be roundabouts, but all of that is expensive. Several of the roads that cross it have only stop signs at the crossing road, and it can be perilous to make a left turn from any of those onto 10 or to go straight through on the crossing road.
35/115 had similar problems. When they built the central barrier many years ago, this blocked going straight or left from such cross roads, so they built bridges to allow that traffic to go over without having to deal with the 35/115 traffic. Not cheap, but no more head-on crashes and fewer crashes at junctions.