But the really important question remains.Leozak" was probably seen as a more "concise" spelling of "Leozack." Sounds the same, but it's one less letter and someone may have thought it looked tidier.
"Zack" is also a common variant of the name "Zach" and I can see someone at Hasbro not wanting Transformers to have human-looking names, so "...zak" is preferable to "...zack."
Is he, or is he not, a Lego maniac?