This is according to a report from Deadline in which while it has yet to be made official, they report that the series’ showrunner and writer, Jane Goldman, had been emailing the cast and crew of the project to let them know that it had been cancelled. It is unclear as to what might have led to the cancellation given how much interest there has been for Game of Thrones.
However, there have been reports about issues arising during the filming process, and how the pilot was apparently not that well-received and had to be re-edited. As Deadline points out, this is not entirely unheard of as the pilot of the original Game of Thrones was said to be quite a mess as well.
It is possible that maybe this was simply more than what HBO was willing to spend which ultimately led them to cancel the series, but either way, it looks like fans will have to hope that George R.R. Martin will come up with books related to the prequel as it seems that we won’t be able to watch it.