T3sl4co1l is perfectly correct on all points other than two.
Firstly, cost. The only real cost is actually China Post when you need to get your shitty large inductors shipped. They're quite heavy. And your external switch if you want some serious power output. Really though you can build the entire supply for $0.15 easily. I doubt you can find a single IC that fits in that at volume cost.
Secondly, stability and ease of use. The 34063 is unconditionally stable. It is pretty difficult to get it to not work with the crappiest board layout, the crappiest caps and the crappiest source impedance. Some of the nicer ICs are terribly tetchy with certain loads. As an example, try driving a load on the end of 6 feet of wire with an LT1073. It WILL oscillate unless you stick a quite frankly very expensive tant on the output. 34063? Meh just works with a shitty $0.01 electrolytic.