When Yakov gave his blessings to his sons, he singled out Reuven as פחז כמים, which is loosely translated as one who acts too hastily.
He made a bad judgement call with the incident of Bilhah which caused him to be absent when Yosef was sold by his brothers. Instead of saving his younger brother, he was busy with his sackcloth in repentance for his Bilhah sin.
Such behavior did not make Reuven fit to be king. A king had to know how to act in a thoughtful and careful manner, before making decisions. This was where Yehuda had far surpassed Reuven in his royal worthiness and leadership.
It is a good lesson for us to learn how to take great care before making major decisions in life. A thoughtless, hasty, decision could have terrible consequences for which we may end up paying for, our whole lives. A good lesson learned the hard way from Reuven.