We tend to see change as risky. While we know what we're going to get from the status quo (both good and bad), we're less certain what we could get from change.
Change creates new situations, and it's difficult to anticipate what those new situations will bring.
A writer will invest lots of effort into a draft. There will be strengths and weaknesses of that draft, but we know what we have, and we know we can continue to try to push forward on it. Because of all the effort that does get invested in a draft, we become attached to it--we like its strengths and its weaknesses are familiar. The idea, therefore, of scrapping the draft and starting anew is intimidating. Will we have a new draft with strengths equal to the last? Will we even have the energy to complete a new draft? It all seems very risky.
Of course I write this to suggest the importance of being able to change, of being able to scrap that old draft to start fresh. I don't think that the effort on the old draft was wasted: we learn from writing that draft; we learn what didn't work.
It may feel like a huge risk and a huge loss of invested effort to decide to start a completely new draft from the blank page. Making a complete change in a work in progress feels like a huge risk. But actually it's not. First off, we're only talking about an investment in a piece that isn't working, a piece that is problematic. Secondly, if the old draft does support what we're trying to accomplish, then once we get going on the new draft it should actually be pretty easy to incorporate sections of the old draft--if not verbatim, at least with only some editing.
We only think about change when we're not satisfied with out situation. For example, I would suggest changing a draft that has been rejected, but I would seriously question the need to change a draft that has been accepted. Given that premise, change really only incurs one additional risk: the risk of uncertainty. Otherwise, the worst change can do (at least as a writer) is leave you exactly where you were: with a draft that is not accepted.