That’s an interesting question, and I think it has to do with him at times being more helpful as a supporting character than the main star of the show.
For example, you can do so much damage sneaking up behind people > stunning + rage > melee huge damage > finish them off with a perfectly placed axe throw as they flee. That burst and the skillful axe toss at the end are some of the most rewarding moments in the game. Especially if you follow it with the ult rune and take focus from the whole team and use them to heal you up. Or even later game where he can go toe to toe in the ring against the Sentry by himself.
BUT
A lot of what makes him great is his behind the scenes kind of work that doesn’t really receive much glory in-game, or in the post match results.
For example, he really shines as a team player in moments like getting and holding the Sentry’s focus in incursion to protect minions and ally attacks on its shield/hp. Or dashing through the corridor on overgrowth and coming up behind the whole team and disrupting/scaring their push with your rune active and taking their heat. A lot of those great team plays involve a good use of shielding and harrying which doesn’t ‘feel’ like you are doing a lot.
So, long story short, to answer OPs original question, even though he can get a fantastic K/D, I think he is played by few because at his best and most team oriented your influence on the game (even though it is great) is implicit, and you sacrifice your glory in doing so.