There are a couple reasons it should be 3 slots.
1. It allows the user to have the best of both worlds. You can carry a great support weapon along with a great dueling weapon.
2. It puts it way out of balance with the long maul, which is a similar weapon.
Let's compare:
Long Maul
weapon length: 125
weight: 7
difficulty: 18
speed rating: 70
weapon length: 125
thrust damage: 10 blunt
swing damage: 37 blunt
slots: 3
Can't use on horseback
Crushthrough
Unbalanced
Knockdown
Great Maul
weapon length: 68
weight: 8
difficulty: 20
speed rating: 80
weapon length: 68
thrust damage: 0 pierce
swing damage: 43 blunt
slots: 2
Can't use on horseback
Crushthrough
Unbalanced
Knockdown
Great Maul has higher difficulty, but the Long Maul barely crushes through anything with only 18 str.
Great Maul is heavier which gives it a higher chance to crush through.
Great Maul is 10 speed faster which makes a big difference at such slow speeds.
Great Maul does 6 more damage.
Long Maul is 125 length and great maul is 68.
Now if we look at the lengths along with the animations, it comes out to this:
Long Maul
Overhead 125 - 15 = 110
Left Swing 125 - 7 = 118
Right Swing 125 - 2 = 123
Great Maul
Overhead 68 + 15 = 83
Left Swing 68 + 17 = 85
Right Swing 68 + 13 = 81
They're a bit closer than, but Long Maul still has the advantage length-wise. However, the longer length of the long maul gives it a shorter sweet spot. You need to be a certain distance away to reliably crush through. Since you can be closer with the Great Maul, that isn't much of an issue.
I'm not saying that each one should be nerfed or buffed, I just think that making them both 3 slots would bring them into balance. The each have their roles. I just find it odd that the Long Maul is 3 slots and the great Maul isn't.