Jonathan Strange and Mr Norrel starts out with the question. The way it is phrased seems to be setting up the rest of the book to answer this question:
It was the most commonplace question in the world. It was the question which, sooner or later, every child in the kingdom asks his governess or his schoolmaster or his parent. Yet the learned members of the York society did not at all like hearing it asked and the reason was this: they were no more able to answer it than any one else.
I don't think the question is ever fully answered in the book. Is that intentional? I'm hoping someone can help me clear up this ambiguity.
- Why did virtually all practice of magic in England stop at some point?
I don't think you can answer this question without also answering:
- What is making it come back?
The Raven King took magic with him when he left, but now he is coming back, and the magic is coming back with him
The movie implies that John Uskglass left England 300 years ago, and took magic with him. Since he is coming back to England, magic is starting to come back with him.
Is this explanation supported by the book?
Vinculus claims that Uskglass is using Strange and Norrell as his puppets to bring English magic back. But he doesn't say why Uskglass took the magic away.
After his interview with Drawlight in the darkness, Strange "channels" Uskglass or something and sends a bunch of ravens to England, which wake up the old alliances, and strange an magical things start happening everywhere.
Just before waking up the old alliances, Strange makes the discovery, and sends word through Drawlight, that
All of John Uskglass's old alliances [that make English magic possible] are still in place. I am sending messengers to remind the stones and the sky and the rain of their ancient promises.
But if the old alliances were still there, why did they require waking up? And why did they fall asleep?
We gradually find out, throughout the book, that there actually have been other people performing magic before the official "Restoration of English Magic", such as Childermass's cards of Marseilles and the ladies of Grace Adieu. This seems to support the notion that magic was possible all along.
- According to this timeline, the Raven King left England in 1434, and there were several other magicians after him, including Martin Pale and Paris Ormskirk. The books suggest that magic declined around 1600 ("the last two hundred years", not 300), which leaves more than 150 years of healthy English magic without Uskglass.
- By the end of the book, there is plenty of magic and magicians all over England, but no Raven King, other than a couple cameo appearances. These appearances can't constitute a "return". Childermass says that there are many stories of visits by the Raven King over the years, and not all of them are incredible, particularly the tale of the Newcastle glovemaker's child. These prior visits by Uskglass did not provoke island-wide magical feats.
Magic was possible all along, but English magicians have been lazy
Norrell says something to this effect in the first episode of the TV series. If so, why were magicians lazier in the past few centuries than in the centuries before, and why are they more energetic now?
"The belief that all practical magicians must be charlatans arises from the shocking idleness of English magicians in the last two hundred years."
@Adamant adds, "I don’t think we can take Norrell’s word on the reasons behind the decline of English magic. He’s shown a distinct tendency to dismiss anything associated with the Raven King, so naturally he cannot be trusted on the reasons for the disappearance of the old magic. He would rather presume that the fault lies in the magicians themselves, rather than appeal to the disappearance of the Raven King, fairies, or anything else 'improper.'"
Magic was possible all along, but books of magic are so rare that no one can learn magic
From Segundus's initial interactions with Norrell, it is implied that magic has been possible all along, if only one has the proper books and enough time to study. Segundus wasn't able to do magic because he only had books about magic, not books of magic.
- Strange's lack of books limits the magic he can perform, but doesn't prevent him from performing magic completely. Spells that didn't work for Segundus, Strange is somehow able to get to work. Why haven't there been more people like him?
- Norrell has been collecting books for a long time, but not for 300 years. The Hurtview library is enormous, and previously the books in it were scattered across England. The York society has access to "five works...which might reasonably be claimed as books of magic", and one might expect to occasionally see "two or three in a private library". Why were these books not used to perform magic before Norrell collected them?