So Why did the first Order in Star Wars build the gorilla walkers and just not use the imperial model At-AT? Any answer is acceptable.
This is addressed in the film's Incredible Cross-Sections factbook. There are a number of reasons why the First Order replaced the AT-AT with the AT-M6 but it boils down to it being a fundamentally upgraded model with better armour, more powerful weaponry and a psychologically more imposing look.
A TOWERING MACHINE seemingly plucked from nightmares, the All Terrain MegaCalibre Six brings devastating firepower to the surface of Crait. Sheathed in state-of-the-art armour forged in secret facilities in the Unknown Regions, the massive AT-M6 is simultaneously a brutally effective siege engine and a fiendish example of psychological warfare. It is a menacing symbol of an emboldened First Order finally unleashed to wreak havoc on the galaxy that rejected its Imperial predecessors. The goal of such an obscene display of murderous power is to reduce enemies to abject terror, incapable of any course of action except total submission.
The AT-M6 is fundamentally a platform for the MegaCalibre Six turbolaser cannon, which dominates the walker's massive fuselage. Intended to make siege warfare simple and short, the M6 is powerful enough to punch through shields rated to deflect bombardment from orbit. Bringing the destructive power of a battleship to ground engagements requires a dedicated power plant and a string of auxiliary fuel cells to reduce the cannon's recharge time.
First Order researchers solved the challenge of how to incorporate the MegaCalibre Six cannon into a walker frame by redesigning the vehicle's front legs. The legs were modified to bear increased weight and to provide the stability needed to brace the craft against the cannon's massive recoil. The vehicle's "knuckle-walking" locomotion has another advantage: it gives the AT-M6 the aspect of a giant simian predator, making an already intimidating weapon even more fearsome to enemy eyes.