Because they want the revenue from new jersey sales, so they need to trot out a new one every year for fans to rush out and buy.
There should be a rule preventing teams from changing jerseys for a 10 year period.
As Leaf fans we're lucky because all of our third jersey's have at least been pretty nice, but this has been the cause of some of the godawful third jersey's we've seen around the league over the years.
Has anyone seen the Islanders new jerseys this year? Brutal.