Because Maokai is a persistent crowd-control threat, and a powerful, sustaining tank to boot. And, according to the Mid Season teaser, his Saplings are going to be dangerous when thrown into bushes.
Here's the best way to look at it: Yes, they are ostensibly similar, but Maokai is a disruptive Vanguard Tank, and Nami is a lightweight Enchanter/Controller Support. Nami's ult is primarily for peel or teamfight disarray, and the rest of her kit is basically utility - increasing ally movement speed, empowering ally auto-attacks, a bouncing heal, and a ranged AoE stun.
Maokai, on the other hand, has his ultimate primarily meant to lock down an enemy team to help him and his teammates reliably keep on them (although it can also be used as peel). Maokai's strengths lie in staying neck-deep in the enemy team, rooting and knocking around enemies so that the rest of his team can capitalise on the chaos.
Maokai is built for lockdown, Nami is built to support.