It's almost like AWS released what they thought was the best product at the time, regretted some of their decisions and then launched other product(s) to replace it....
In this case, it seems like the one-size-fits-all ELB has been replaced by ALB for those using containers, who want L7 LB, and don't need insanity-scale. NLB for those who want massive scale, a dumb pipe and/or need consistent IPs. They could have tried to build these features into ELB but they didn't, they deliberately created new nomenclature to get rid of baggage.
Also see: SimpleDB -> DynamoDB. EC2 Classic -> VPCs.