There are a few reasons for supporting only one deployment per environment at the same time:
Pega import process is not thread safe, so if same rule is modified during parallel deployments (application imports), deployments could fail and it will be very challenging to identify which deployment was the source of the failure.
Before each deployment, a restore point will be created on that environment. So, in case of parallel deployments, if we are not sure which deployment have caused the failure it again will be challenging to identify which deployment restore point to use .
You could of course have multiple, different, pipelines deploying to the same environments, however keep in mind again about the challenging with doing parallel imports on the same environments. If the same set of rules are being imported, there could again be failures that are hard to diagnose and fix.