Making something people want is a necessary (but not sufficient) condition for start-up success, and it's one that I see tripping up early-stage startup founders the most. Its incredible how many people build in a vacuum for 6+ months, and then release an overly-complex product that doesn't address a single real customer need.
Being able to rationalize "why now?" is much easier after the fact, as the OP demonstrates with his list of his now-successful companies. Doing the same looking forward is much trickier.
I agree, and think the retrospective analysis on this is often really sloppy. In hindsight, the iPhone was clearly at the right time, and the Newton was clearly at the wrong time, but the track record of anyone distinguishing between those is very poor. Even Jobs, famous for some of the big wins on that score, was wrong about timing as often as he was right (e.g. NeXT was about as timely as the BeBox with its initial launch).
(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.
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.
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.
I think local merchants desperately wanted a way to use social networking to connect with customers and get them coming back. My local sushi place has a deal, check in 3 times and get a free appetizer. Users wanted a new, fun way to use the GPS on their smartphones, and they were looking for something more engaging than Twitter or Facebook.
I can say with 100% certainty that Foursquare addressed both needs nicely. I'm not a Twitter user, but I used Foursquare religiously in the early days. I spent more time (and money) becoming a regular at said sushi place just to defend my mayorship.
Foursquare tried to make them want it, and retrospectively it looks like people did. The decreasing number of checkins on that and Places would suggest that it didn't work, because presumably people didn't actually want it and they weren't convinced enough to use it.
Edit: in fact, it's kind of a perfect example of 'why now' making it seem like it makes sense, and bandwagoning, and the appearance of 'solving a problem' and 'changing the world', all to little effect because it's a shakey idea to begin with.