Apple only started doing these maps in 2011, when it got the technology from a Swedish company that it bought (C3 Technologies). And it has only very recently (since Oct 2012) produced the new enhanced visuals for its maps. So this is all very much still evolving, and we can expect to hear a lot of news in the next months.
Meanwhile, the technology is stll having some teething troubles and is not available in all countries yet - you can see a list of what's available here.
The intention is certainly for the iOS6 and all its features to be available in all countries, though there doesn't seem to be a timetable for this. As for a flyover of every town on earth, this has not publicly been given as a goal, but Apple definitely wants to expand coverage as much as possible. You can be sure that all major cities will soon be covered at least, if nothing happens to interfere with Apple's plans.