Simple difference of views. Stan was a writer, so he had those things on his mind. In his mind, you come up with a character and a story, then you worry about making them look cool later.
Liefeld and MacFarlane are artists, so they work going the opposite direction. Give the character a cool look and name, and then figure out the rest later.
Also, while the character is popular now, I hesitate to automatically call a character like Deadpool iconic when he's really just Liefeld really wanting to draw Deathstroke despite not working at DC. Not to mention that it didn't really take off until years later when writers decided to make the character a parody of itself and overly serious comics at the time.