NiMH may have been the current battery trend when this thing was manufactured. Remember that NiMH had started replacing NiCd technology and the lithium chemistries were still problematic with cost, explosions, improper charging, improper discharging, etc. for several years after NiMH was developed before it finally became feasible and cost-effective. Although the AA package would have provided the higher power needed, it may have been much too thick for the application. Remember that designers like to build around required features and not available battery sizes. That's why you often get ******* battery sizes in your digital cameras and such.
Dean