A few years ago one of my neighbours ended up with a blockage in the sewers that were shared between the house. it was coming up through the drains in a couple of the gardens including mine, and as this was a health hazard, the local council came out for a look, and managed to get it unblocked (turned out a family further up the street were flushing disposable nappies down the toilet!).
I had a bit of a chat with the man from the council, and turns out they were really busy all the time now with un blocking regular drains, that had nothing in them other than what you'd expect (i.e. no nappies this time).
The primary reason he gave was that most of the sewerage infrastructure (most of which is quote old now) was designed to have an expected amount of water flowing in order to keep it clear. With the advent of metered water supplies, and water saving toilets, there was no longer enough water going through the system to keep things moving, especially in dry spells, so no rain water either.