Everytime an array element changed position, not only it's index changes, but a lot of--and sometimes all--other array elements indices change also. That would require iterating the hash (via the array of keys) to adjust all the embedded indices.
It's not the complexity that is the problem, it is the time element.
Examine what is said, not who speaks.
"Efficiency is intelligent laziness." -David Dunham
"Think for yourself!" - Abigail
"Memory, processor, disk in that order on the hardware side. Algorithm, algorithm, algorithm on the code side." - tachyon