Because they have matured, their businesses have stabilized & their services became a commodity (Just like Electric Utilities) and they are also heavily regulated, so they don't have much to do in terms of growth.
In business terms, they have turned into "Cash Cows", you don't expect them to generate huge growth but you would expect an excellent amount of cash generation & distribution.
Also this industry is very intensive in Capex (Capital Expenditure), so you don't expect competitors to enter this market easily and grab some market share, won't happen.