Its because the only character they ever BUILD on is Zero. he gets ysterious dreams of his past, he gets love interests and friends whom he later is forced to kill for the mission, he gets used, he he he.
X? Alia obviously has feelings for him, but hes as oblivious as can be, his character doesnt really go anywhere, his highlight was when Double betrayed him, that was just about it, they tried to give him some oomph to his character by adding his "dilemma" over violence, which resulted in the forced character development called X7, where he retired from active hunting. Which didnt feel natural at all (and was contradictory to it being an X series game, not a Zero/Axl one, wtf the main character is an unlockable?), and actually felt out of character. Hes always disliked having to resort to killing, but much like Zero, if he had to, he did it. It never caused him THAT much inner turmoil. He was basically just the sappy equivalent to Zero. and then suddenly in X8, hes back to his usual self, as if nothing happened. He moans and groans about the fighting starting again, but thats it. he actually seems almost too comfortable with maverick hunting, more than usual.
In short, Its because Zero gets the better Character development, while X gets almost none.