Or, another way of looking (simplistically) at it, as the tip is in the vortex, and producing no lift, you can't actually stall it. Therefore the rest of the wing will stall, and still the tip won't. Again, if you look at the lift distribution over the wing, it is maximum at the centre, and minimum at the tip. As the centre bit is 'working' the hardest, it will feel the effect of increasing AOA the most, and so on out to the tip. The explanation won't be liked by the experts, but it does illustrate why the stall is progressive from centre to tip.
Evan, WB #12.