Why were the AT-ATs destroyed so easily in this film?

Rating: Null/10 (7 April 2017)

They were AT-ACTs (All Terrain Armored Cargo Transport). These were not designed for attacking, but transporting supplies and likely shipments of Kyber crystals for the Death Star. So it's likely they were not as heavily armored as the AT- ATs. In addition, transports were going up against starfighters and Imperial surface-to-air missiles rather than snowspeeders.