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For an ALB - "You must specify subnets from at least two Availability Zones."
For an NLB - "You can specify subnets from one or more Availability Zones."
What is the reason for enforcing 2 AZs as a MUST requirement for ALB specifically and not for NLB?
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You can create a VPC that spans multiple Availability Zones. After creating a VPC, you can add one or more subnets in each Availability Zone. Each subnet must reside exclusively within one Availability Zone and cannot span zones.
The load balancer must have at least one subnet at all times. You can add at most one subnet per Availability Zone. You cannot add a Local Zone subnet.
Subnets for your load balancer. When you create an Application Load Balancer, you must specify one of the following types of subnets: Availability Zone, Local Zone, or Outpost. You must select at least two Availability Zone subnets.
ALB requires at least two subnets across Availability Zones, NLB requires one subnet.
The Network Load Balancer is designed to honor zonal isolation requirements, and thus is suitable for application architectures encapuslated in a single zone. Although, if the Availability Zone fails, there is the option to automatically fail-over to other healthy AZs.
In their guidelines AWS recommend customers configure the NLB targets in multiple AZs for high availability, but it is not a must.
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