1. Neither are particularly interested in revenge at this point.
2. Obi-Wan is for all intents and purposes a hermit; Maul is still active in the galaxy and is getting directly involved with the main cast. I, personally, can't justify him ending up on Tatooine chasing an old enemy he doesn't even have current beef with.
3. Maul has had ample opportunities to kill Obi-Wan. TPM. All through TCW from his return, during both TCW tie-in comics they appeared in, up to the Mandalore arcs.
But character motives, or as we know it, PIS, has always prevented him from killing Obi-Wan. Obi-Wan in contrast has never had that kind of ability or resourcefulness. He got lucky once because Maul was gloating. For Obi-Wan to kill Maul there would need to be an element of bullshit I'm pretty tired of seeing.
I want Maul to be killed fighting an equal or stronger opponent; just a simple, badass fight. No bullshit with Maul slipping on a banana peel into Kanan's lightsaber. It's not really asking for much; let Vader or Sidious do it.
Whatever he's been up to--it has likely been part of Palpatine's plan: http://i.imgur.com/zHe4kWq.jpg
...Or at least, that's what Palps alluded to. /shrug
"A great tactician creates plans.
A good tactician recognizes the soundness of a plan presented to him.
A fair tactician must see the plan succeed before offering approval.
Those with no tactical ability at all may never comprehend the plan, and will resent the tactician."