The number of attacks per character was made to be simple like in Smash bros. There aren't enough options to make a full robust attack/combo system, but it would be needlessly complex, especially if the game had 4 player combat.
With only two attack buttons (attack and special), you could have a lot more attack options and still keep the combat relatively simple.
z: attack 1
z+u: attack 2
z+d: attack 3
z+l: attack 4
z+r: attack 5
c: special attack 1
c+u: special attack 2
c+d: special attack 3
c+l: special attack 4
c+r: special attack 5
z+c: super special attack 1
z+c+u: super special attack 2
z+c+d: super special attack 3
z+c+l: super special attack 4
z+c+r: super special attack 5
z+x: grab 1
z+x+u: grab 2
z+x+d: grab 3
z+x+l: grab 4
z+x+r: grab 5
z (in air): air attack 1
z+u (in air): air attack 2
z+d (in air): air attack 3
z+l (in air): air attack 4
z+r (in air): air attack 5
c (in air): air special attack 1
c+u (in air): air special attack 2
c+d (in air): air special attack 3
c+l (in air): air special attack 4
c+r (in air): air special attack 5
z+c (in air): air super special attack 1
z+c+u (in air): air super special attack 2
z+c+d (in air): air super special attack 3
z+c+l (in air): air super special attack 4
z+c+r (in air): air super special attack 5
z+x (in air): air grab 1
z+x+u (in air): air grab 2
z+x+d (in air): air grab 3
z+x+l (in air): air grab 4
z+x+r (in air): air grab 5
z (during dash): dash attack
c (during dash): special dash attack
z+c (during dash): super special dash attack
//plus here's a list of possible movement commands
r, r: dash
x: block
x (in air): air block
x+r: forward dodge
x+l: backwards dodge
x+d: parry
x+u: counter
j: jump
u: jump
j (in air): double jump
u (in air): double jump
d, u: super jump
u+j: super jump
j+d: drop