Why trilogies and sagas?

It occured to me actually today that in the past few years especially trilogies and sagas were very poular and successful among young readers. Talking of which Harry Potter, His dark Materials, The Lord of the Rings (which is matter-of-factly no trilogy but one volume with seven books), The Twilight saga and The Hunger Games Saga are only a few to name.
But what is it that makes a long story so different to a short read. Is it the suspension waiting for the follow up? Is it the way characters, places and plots can be shown in their very detail? Is it simply a story too long to squeeze into one single book? Or a clever book selling strategy? All - except the Hunger Games saga (more than 26 million copies sold world wide)- had their movies followed, which were a huge success too. So of course there is a lot of money involved but I don't think that Mr Tolkien was thinking of a major success and loads of money when he wrote The Lord of the Rings. It's rather this explenation which makes sense:
Trilogies — and series in general — are common in science fiction and fantasy because of the artistic importance of complex ideas.
Yes, all of the above named books are science fiction or fantasy but I'm sure there are non-science fictional trilogies out there as well. Per definition a single book of a trilogy can stand alone for itself but I reckon everybody admitts especially in the Harry Potter case this is not entirely true. And it is also not true for  Tolkien's Hobbits wondering through Middle-earth.
Why don't you write a list of the top trilogies you read?

Mr Schmidt

Here's my personal Top 3:

The Hunger Games
His dark materials
Lord of the Rings