Why we can't use AHB in the Cortex M0 MCU?
Which interconnect are you using? You should find that a multi-layer AHB can be driven from AHB-Lite, either with some tie-offs, or using a bridge.
As to why, it is to avoid having extra logic in the core which isn't needed for the majority of use-cases. Adding the logic to support arbitration is no less efficient when it is external.