The reasoning behind this is that different tenants may have different deployment targets within the same environment.
e.g.
You’re deploying a website and have two customers using that website. Each customer’s site is customised slightly requiring them to be hosted on different servers.
So, you have two tenants, two deployment targets (webservers), and both of these targets sit within the same Production environment.
Without being able to specify which tenants the deployment target belongs to it would be necessary to create a separate environment for every tenant.