Because it very obviously didn't fit any criteria for murder and he wasn't charged with causing her death at all.
They argued, he got in his car to leave so they could cool off. She jumped on top of the bonnet of his car as he drove off and laid there to try to stop him leaving - he continued to drive at about 20mph for 6 seconds instead of stopping immediately. Then he stopped the car, she jumped off the bonnet and as he started to drive off she opened the passenger door to climb in after him. He reached over and shut it, then drove off. These parts were what he was charged with and pled guilty to - not stopping immediately when she jumped on the bonnet and accelerating while she was standing so close to the car.
She then grabbed onto the outside of the moving car and ran alongside it 2 seconds before tripping - this part was captured on CCTV. He knew she was next to the car because of what had gone on but couldn't see that she had decided to grab on. When she tripped she fell partly under the car and was hit. He immediately stopped, called for help, and tried to administer aid until the ambulance got there.
It's absolutely tragic, particularly for their children. But it wasn't murder, it was an accident that came from two adults who let their anger make them act recklessly.
If the sexes were reversed and a woman was trying to flee an argument with her husband while he tried to physically stop her would you want her really charged for his murder?