Because in this league, you win when you have your best players signed at way below market worth. There's no two ways about it. Look at all the Cup winners over the last few years and examine how much $ their star players were making; Most of them were underpaid relative to their market worth by either being on an ELC, being given a long-term second deal or being one of those (now illegal) cap circumvention contracts. Look at the majority of the top teams in the league and look at how much their star players are making relative to their production.
Look at just about every young star player in the league. Tavares, Stamkos, Doughty, Seguin, McDonagh, Toews, Kane, etc. Those guys all signed a long-term deal after their ELC was finished and they were signed at below market value. $5.5M AAV over six years for John Tavares? $7.5M AAV over six years for Steven Stamkos? Are you kidding me? If Tavares & Stamkos were UFAs tomorrow, how much do you think he'd get from their respective teams? $11M? $12M? $13M? Toews & Kane were making $6.5M for six years, allowing Chicago to get good depth players to surround them with the money they saved. Now that they're both making $10.5M, Patrick Sharp is gone, Johny Oduya's gone, and they're hoping the cheap alternatives can step up. Tyler Seguin makes $5.75M for four more years.
With a hard cap, teams with the best "bang for buck" contracts win and the math is very simple; It's impossible to argue that Subban at $5M to $5.5M over 5 or 6 years (depending on what reports you read or believe) isn't a better deal than Subban at $2,875M for two years then 8 years at $9M. You just can't. Especially when you consider this team finished dead last in the East the year before his new contract kicked in. It's not like this was a contending team and we had to get Subban on the very cheap to go for it. The deal was sheer idiocy and it is holding this team back.