Because Konohagakure-Arena-logic doesn't apply in a manga fight. That's where most people fail, when talking about Itachi>Obito or Itachi<Obito.
Obito can
teleport away whenever he wants and this is what makes it so difficult to actually defeat him. Not to mention that he could and would throw the whole Akatsuki (hell, Pain would be enough) at Itachi, if he actively tried to find and kill Obito. Itachi didn't have the tools to track down Obito and fight him in a fair fight one on one, that's something most people forget and this has nothing to do with powerlevels.
Obito on the other hand stated he would be dead, if Itachi had known about his secret and Itachi's abilities proved that he is a real threat to almost anyone. One mistake on Obito's side and he gets blitzed by the Sword of Totsuka and it's debatable if Obito can use Izanagi while pierced by this sword. We have two examples in Orochimaru (Sannin) and
Nagato (Rinnegan-User), who were unable to do anything after being hit.
Izanagi? The moment Obito uses Izanagi and fails to kill Itachi in his first attempt, he will be in deep shit. Because all Itachi has to do is to activate Izanami with one eye. Then he has to fulfill the two "conditions". Funny thing is that it's highly possible that Itachi can also use Izanagi. Which means, Itachi has to fulfill one "condition" and can take a fatal blow from Obito in order to fulfill the second "condition", use Izanagi and right after that cast Izanami.
Outcome of this scenario? Itachi blind in both eyes and Obito dead.
We have two good examples about how big of a threat/hindrance Itachi was for Obito. The first one
Obito himself believing he would be dead, if he had known about his secret(s) and even when talking in private with Zetsu, Obito states that with Itachi's death,
nothing stands in his way now.
If taking down Itachi was an easy task, you can be sure that Obito would have done it already. You believe someone like Obito really cares about deals?