Some of the reasons are;

It being a page turner. There was no boring moment. Things happened all the time.
The manner of narration was so concise, to the point, and apt.
The description of Matilda's death - She expired.

It remimded me of Margaret in S&S2, as she comes running and tells Elinor and their mother breathlessly, 'She fell down and he carried her.'
So according to Margaret the story could go something like this,
"It killed him and he accidentally killed her. The ghost grew too big, and told them who was the rightful prince, and he and she went to live in the monastry."

***sigh** Now that we have finished TCoO I have to go back and finish 'Clermont' with all those forests and verdure on the slopes of hills, and their endless description. ***sigh****

Well another reason was the humour.
Out of five chapters at least three were quite funny - the first, third and fourth. In some parts also the fifth, before the tragedy.

I think that starting a trend which continued in the later novels is quite impressive, and to this day we have cowl-wearing apparitions/sinister figures like in that series about the three witches (can't remember the name, my daughters watch it now and then).

Did anyone else enjoy the book?