Nailgun Massacre

At a construction site in east Texas, six men gang rape Linda Jenkins.
Five months later, a person in camouflage clothing and black motorcycle helmet kills one of the rapists with a nail gun. One month later another rapist, Mark, cuts some wood with his friend Brad. As Brad urinates, the killer shoots him in the crotch and stomach. As Mark revs up the chainsaw, the killer sneaks up behind him and shoots him in the back of the neck. Mark falls and the chainsaw severs his hand.
When Doc, the town doctor and coroner, arrives at the crime scene, the sheriff shows him the bodies. The doctor mentions that three bodies have been found on Old Lady Bailey's property. The sheriff asks the wife of the first murder victim if her husband was a carpenter, and he leaves to call the "meat wagon."
The killer picks up a hitchhiker, then shoots him in the stomach, causing him to jump out of the moving vehicle. He then stops the car and kills the hitchhiker. The sheriff and Doc are called to the hardware store where a young woman was found nailed behind the store. Minutes later, they are called to the road outside of town where the hitchhiker was nailed to the concrete in the middle of the road.
Maxine, John and Tom are eating at a diner where she reveals Old Bailey let them stay in a house for free because someone was murdered there. The next day, the trio buys lumber from the lumber yard to fix the house. Hal and Ben arrive with their girlfriends Ann and Trish, to ask the owner, Bubba, for a job. He tells them of Maxine's place and the group leaves. After they leave, Linda shows up and asks Bubba if the two men were construction workers.
At Maxine's place, the group sees no one at home, and sit on a blanket for a picnic. Hal and Ann go for a walk in the woods and have sex. They are killed and nailed to a tree by the killer. Trish urges Ben to look for them. The killer nails Ben's hands to a tree. Maxine, John and Tom returns to the house. John comforts a frightened Trish. John and Tom discover the Hal, Ben, and Ann's bodies and call the Sheriff. They unload the lumber from the truck and Maxine voices her worries of the killer. They also find their nail gun missing.
The next day, two carpenters are working on a house when they playfully shoot at each other with nail guns. When they go back into the partially constructed house, the killer shoots one in the head. The killer, speaking with a distorted voice, tells the other carpenter to think back six months ago. When the carpenter mentions the raping, the killer kills him. The killer then murders a couple making out on the hood of their car, and a family man in a suburban house.
After following various leads, the Sheriff eventually realizes six of the victims worked at a construction site. Six month ago, Linda Jenkins, now established to be Bubba's sister, was raped after delivering supplies at the site. The six rapists walked away thanks to the lack of evidence. That evening, two women walking in the nearby woods are attacked and killed.
The next morning, Doc calls Linda and asks to speak to Bubba, but she hangs up. Doc drives over to the lumber yard and confronts Linda, who tells him Bubba is out driving his hearse. Doc and Linda drive along the road and find the hearse and give chase. Soon, the sheriff joins them and the chases lead to a textile factory. The killer exits the car and runs, pursued by Sheriff and Doc. They chase the killer through the factory to the limestone mounds. The killer climbs up a crane, but slips and falls to his death. Linda walks up to the dead killer and removes his helmet, revealing him to be Bubba. The killings now over, Doc walks away with the traumatized Linda.

Question: Why were the rapists not punished?
Answer: Lack of evidence