I think it's a priority and resource issue. Right now their priority is constantly tweaking the automated system to be able to evolve and detect more negative behavior. I remember hearing they are also looking to launch a new Tribunal (player-side) either with or soon after the new client.
As far as encouraging positive behavior I'm sure it's something they want to build on, but they have to figure out a worthwhile incentive for long-term positive behavior. Right now Honor gives a tiny ribbon based on an arbitrary calculation that nobody knows and actually wears off over time. On top of that different honor counts for different ribbons, meaning if it's too spread out I believe it can actually hurt your chances. For many it is not worth it. I remember in previous years they gave out skins and icons (like Santa Baron) during the preseason for people who never got punished the previous season (and I'm sure played a minimum of games) but again that's not really Honor-based as much as Tribunal-based. The problem becomes how do you give out a worthwhile award tied to Honor that people don't instantly try to reverse engineer and grind out?
I am confident Riot and the PB&J team (is that still the nickname?) want to incentivize positive behavior, but it would require reworking the Honor system and frankly I think they want to finish the Tribunal and automated punishments first.
