(Obviously) I don't think asking why now is the right time for your idea is premature. I think it's orthogonal to the product building piece. I think a lot of YC companies could avoid a lot of pain if they asked this question early on, because it would lead them to talk to others who have worked in the space. Often they will quickly discover that the fundamental dynamics of an industry haven't changed yet and no matter how much time they spend building it's not worth their time yet. At the very least they should be able to point to why the other, previous attempts weren't quite right and why the time is now for this idea to take hold. But all too often companies just start building without understanding why the previous attempts have failed.
On the other hand, one shouldn't underestimate the power of not knowing something is impossible.
If you talk to a lot of others who have worked in the space you may well end up with a long list of reasons why your idea is doomed. It may stop you from wasting time on an idea that can't succeed. Or, possibly more likely, it will muddy your thoughts and prevent you from observing the space with the fresh eyes that are needed to see what those before you have missed.
But all too often companies just start building without understanding why the previous attempts have failed.
Sometimes you need to just start building to begin to understand exactly why previous attempts have failed.
I'm a big proponent of naively trying to do what others tell me is impossible and it's served me perfectly well. That being said, if someone's startup was unsuccessful for a very clear reason that still exists today, the point is to understand that cause and your own plan for overcoming it, or don't start dedicating a bunch of resources only to get to the exact same point and realize it's not something that can be overcome.
It would be like starting a poker website now and completely ignoring what just happened to Full Tilt. Online poker is illegal. Until that changes, probably not a good space to play in.
Figuring out "why now?" implies you understand and can verbalize the exact spot an industry is in. This wil take you at least 4 years of working in hat industry. Infact I would go so far as to say it's nearly impossible to answer the question "why now" without creating a product or working in that industry. "build something people want" is a much better tactic than making decisions based on an answer to this question which in all essence is a thought experiment of dubious value.
(Obviously) I don't think asking why now is the right time for your idea is premature. I think it's orthogonal to the product building piece. I think a lot of YC companies could avoid a lot of pain if they asked this question early on, because it would lead them to talk to others who have worked in the space. Often they will quickly discover that the fundamental dynamics of an industry haven't changed yet and no matter how much time they spend building it's not worth their time yet. At the very least they should be able to point to why the other, previous attempts weren't quite right and why the time is now for this idea to take hold. But all too often companies just start building without understanding why the previous attempts have failed.