You must be joking right??
6061 series used in cheap mountain bikes
7000 used in performance.
Like I said before poor design allied with poor material choice (who knows what they chose) but I bet that the alloy they used was the cheap option.
Nope. Not joking. Its only what you use it for, that determines if a particular alloy is more or less appropriate. I'm not a specialist when it comes to mountain biking, but I assume 7000 series is preferred because it is less ductile than 6061 (less flex), but that also makes it more brittle. Depending on what you use it for however, 6061 may be the better choice. For example, 6061 is the most common aluminium alloy used in the aerospace industry, which it wouldn't be if it was just plain inferior. In fact, that flex which is undesirable for cycling may be exactly what you'd want for wing construction.
While 7000 series aluminium is slightly more expensive, for something like an iPhone frame we're talking about just a few cents, if even that. If mountain bikes built out of 7000 series alloys are much more expensive than their 6061 based counterparts, than I suspect that to be a result of the 7000 series alloys being more difficult/expensive to work with, particularly where welding is required.
Anyway, unless there is some evidence that Apple deliberately chose an aluminium alloy that they knew was a poorer choice for iPhone frame construction, just to save two or three cents per device, I'll remain sceptical. Don't get me wrong. I'm not saying it's impossible. I just don't think it's a conclusion we should jump to.