It's solely for cost purposes.
Right before the pandemic, we played a charity tournament out of the DCU Center (Worcester Railers home ice), and a bunch of Railers players played in it.
Athletic Knit had people there, as they had just signed the contract with the ECHL, and they supplied jerseys for the tourney that we were allowed to keep. We were told these would be exact specs that the league would be using.
They were far and away better quality than any AK jersey I've seen or owned in 20+ years of beer league. However, obviously not near the quality of anything the AHL/NHL has used in a long time.
I highly doubt the NHL would go with an "off" brand for their jersey choice when the next deal is signed.