I would imagine that it's something to do with the fact that the old cast iron gas mains tended to spring leaks eventually. When that happened, eventually something would go bang and when that happened there was a good chance people would die, hence the need to replace the old cast iron gas mains with plastic ones. In the old days when sewage systems were owned and run by local councils, they had a say in the planning consent process. Since 1991 however when the industry was privatised, that has no longer been the case. This means that developers can put up hundreds of houses and just plug the waste water infrastructure for all those new houses into the existing system meaning that eventually it will be running at 100% capacity all the time and when we have, for example a period of prolonged rain, it gets overwhelmed and spills into water courses or the sea. That, and continual cost cutting by the privatised water companies who for the most part are owned by foreign investors means that things are not maintained, are only fixed when they break and that's only the minimum repair needed to get things going again. Often, built in redundancy is allowed to degrade so that plant is reduced to running at 100% and is left with a single point of failure. This isn't confined to our local provider, all the waste water companies are guilty of it and they're all very sorry and promise to improve when they're hauled in front of the courts by the Environment Agency and fined hundreds of millions of pounds of our money.
But never mind, as long as the shareholders get their dividends and the executives their fat bonuses, all is right with the world, right? Plus, it keeps me in a job.