Mainly because he has only been a #1 goalie one season (last season) and goalie consistency is an issue league-wide.
There are only a handful of goalies good enough to sign long term - I think an approach of having two goalies on reasonable short term deals is better way to manage the position.
Plus we already had him signed for next season. I would have waited, made sure he is able to sustain his performance for at least part of the campaign and then sign him if he does.
He'll be 29 in August and this contract will carry him until he is 34. The per annum amount is reasonable, but obviously to get that they felt compelled to give him more years. I guess we'll see what happens and who they use as a backup goalie will tell us more too.