As far as I understand it, Curtis was friends with Leonard Goldberg, who was Vice President of Daytime Programming at ABC at the time, to whom the idea was pitched to. So it's probably just serendipity that Dark Shadows happened to be a soap. I'm sure if Curtis' contact had been in prime-time it would have been made as a prime-time show.
There's an interview with Goldberg on one of the DVD sets, comes across as a real nice person. I love how he refers to Barnabas as 'Uncle Barnabas'. Something tells me he's thinking on the lines of Uncle Fester.