Wand allegiance

[lealess]

No: HPFGUIDX 182046

Could someone explain to me why a wand would not switch allegiance 
from someone who gives himself up to be killed to the killer?  After 
all, it wasn't the soul piece that decided to let Voldemort kill it.  
It was Harry.  That seems a clear defeat to me.  Does the wand really 
care about who he allowed himself to be killed for?  I know there's 
some kind of explanation, probably to do with Voldemort passing out or 
someting, but it keeps eluding me.