Some good poker players only know how to throw a fastball. Even if a fastball is their best pitch, they never become great players because they are so one-dimensional.
Great players learn to throw more than just a fastball.
Some players showdown good cards only. While playing good cards certainly is a cornerstone of winning poker, if you never win pots with bad cards, you simply are not playing very well. If you make yourself so utterly predictable that your opponents can always easily put you on a small range of hands, you make yourself an easier target. Imagine if you faced an opponent who only raises in first position in Holdem with AA or KK, nothing else. Now he raises.