It was essentially because they didn't expect to win, and when they did, they didn't have a plan and didn't have a clear idea of what Brexit was supposed to look like, and they therefore spent the last three years arguing about that. Even at the very, very end when they had to vote, they couldn't agree among themselves what it meant to leave the EU and what their new relationship with Europe should be. I think your point about not having a clear goal is very pertinent in this situation.
On May 2nd, 2019. See this statement in context.