Why Pneumatics instead of servos?
Servos are great. You can buy high-torque ones and fast ones and they are relatively easy to program since they are repeatable (go at this speed to this angle) and work great when doing calculations.
They are too repeatable and they are too easy to program. I know it doesn't make sense, don't you want it to be easy to program? Yes and no. Yes because then development is easier but no because they also don't represent our muscles all too well. Only muscle memory or reaching the maximum stretch/contraction of a muscle/ligament/tendon allow us to have repeatable actions, and this comes with time and repetition, and is still off by a little bit. So pneumatics really fit better when talking about muscles/tendons/ligaments.
Now this isn't to say I won't use servos at all in my design, on the contrary I can see a number of uses already in my head for them, such as move the head around, possibly used for the leg-to-hip assembly and possibly a weight mass in the torso of the robot to better counter-act or
What about McKibben Artificial Muscles?
Sure, I could use McKibben Artificial Muscles, but they are expensive to buy and if I make them myself (which is apparently pretty easy to do) they may not be exactly the same and lead to more programming complications.
Wait, didn't you want them not to be repeatable? Yes, but I also don't want to spend years on the programming side of this (although it may take that long anyways) trying to figure out the differences between the muscles on either side of the robot. Also they tend to wear out faster than a regular pneumatic cylinder.