Because the petroleum ( gas ) was less rare.. Now it's getting rarer and rarer so they are putting the prices higher.. But so are pays and food and everything else.. So it's all around the same price .. Well it's more expensive but in the eighties it would be like 35c a L but they would get paid like 6-8$ a week.. Well I'm not sure if that was in the 80-70-60 or 50's but yeah... I hope I helped :)
People who say it is more rare now are misguided. Yes, there is less on earth, but our access to the remaining deposits has put much more petroleum into the supply chain. This increase in production would in and of itself lower prices, but it accompanies a much higher demand for oil, inflation, and the overestimation of oil's value by OPEC, which produces a majority of the world's petroleum.