Powerful and ancient magic that requires the user to pay a heavy price. A lot of knowledge is kept hidden from your average sorcerer. Ritual magic tends to be either complicated in it's very nature, making casting such spells dangerous - even compared to regular spellcasting.
Its danger can also come from the rather vague nature of it, increasing the chance of an incantation backfiring - sometimes the repercussions being obvious and immediate, other times not so much.