While timing did play a part, I'd suggest it's not so much timing as it is execution that made the biggest difference, in this case.
Android and iOS tablets operate in broadly the same ways as each other and are wildly successful. Windows 8 tablets, which work in much the same way as the Windows tablets that preceded them (i.e. trying to bring the feel of a desktop OS to a tablet form factor), are failing to gain any significant presence in the market, despite having the right timing and loads of marketing. To me, that's a strong indication that the thing holding back tablets prior to iOS and Android arriving was not that people weren't ready for them, but that the tablet concept simply wasn't executed properly.
Same deal with smartphones. Smartphones were around since the '90s, but they only represented an incredibly small portion of the cell phone market. Fast forward a few years, and we get Android and iOS, which, when they first came out, had most of the same features as the smartphones that preceded them, yet they implemented and executed those in a drastically different way that made them much more compelling to users. Blackberry and Palm had the right timing, since they were there from the beginning. What they lacked was proper execution to bring it to the general population.
You're right that there wasn't a market back then, but there wasn't a market because there wasn't a product done right yet. Ideas are cheap. Execution is what matters.