A lot of it just comes down to KISS and avoiding unnecessary overhead and indirection (like vtables, C++ STL containers etc.) so you're getting the most out of the hardware.
It's fair to mention that DOD is not only getting the most out of the hardware. It also allows the busy work to be avoided. I caught myself a couple of times, when I wanted to make a set of types united by some interface. However, in reality what I could do (and I did eventually) is having several instances of single SOA (one per type) which were processed differently. The addition of a new "type" turned from a good hundred line patch to 20-30 lines