I think that you don't read much about Horizon because it is a very stable conservative airline. No big expansion ever, No big aircraft orders, no jumping from code share to code share etc...
As such, upgrade times have always been slow to come and lots of people that work there do so because they desire to live in the Pacific Northwest and are willing to wait for the left seat. I would say that they are a very mature airline where growth is there but it is 2- 4 % a year net. Because of that not many up and coming pilots go there because the need for the left seat is the driving force in their career choice and it takes a long time at Horizon. It is just like a major airline only without the pay, the benefits and the work rules.