Pretty obvious, I'd say, but hey, here's the argument:
1. Obito said Itachi could kill him with knowledge
And knowledge is gained from experience, for those of you who don't know.
So Obito carefully guarding his secrets means he was the one keeping his distance.
2. Why didn't Itachi kill him then?
Why didn't the Five Kages and all of their guards kill Tobi?
Why'd didn't Minato with all of Konoha kill Tobi?
Because Obito can phase out of and teleport away from any fight.
3. Itachi was the one (who couldn't teleport) surrounded by Tobi's allies.
Why didn't Tobi defeat Itachi and control him with genjutsu like he did with Yagura?
Why didn't Tobi store Itachi in the handy-dandy pocket-dimension until he needed him like with Torrune?
Why didn't Tobi replace Itachi with a White Zetsu clone for Sasuke like he did with Kisame?
Why did Tobi hide in the shadows for years when Itachi joined, and didn't emerge until just before Itachi died?
4. Itachi was Tobi's problem.
Regardless of translation specifics, Itachi was a problem.
And Tobi didn't make a move on Konoha until Itachi was dead.
If Tobi betrayed a pact after the pactmaker died, that means he didn't respect the pact.
Only the pactmaker. Tobi didn't move purely because he didn't want to cross a living Itachi.
He had Akatsuki annihilating Konoha before Itachi's body was even cold.
4. Tobi thought Itachi was the bee's-knees. Itachi thought Tobi ['current Madara'] was pathetic.
And you'd be surprised how many can't differentiate 'pathetic shell of former self' from 'invincible Madara.'
5. Tobi's Kamui has an obvious counter.
Tobi cannot attack and defend at the same time.
Gai countered Obito's offense with his nunchucks. It isn't unstoppable.
But guess who has clones? Guess who can fool a Dragon Sage and EMS user with feints?
Guess whose clones explode, which can make Obito absorb explosive damage like Konan did?
Heck, he has exploding clones and one of the fastest ninjutsu and one the fastest jutsu molding speeds.
Itachi's a walking counter to Obito's Kamui.
The author wouldn't even have to create new jutsu out of nowehere for Itachi to look great...
5. Tobi's only other primary jutsu was Izanagi
Who was that ninja with the jutsu specifically designed to trounce Izanagi?
So, why didn't the terminally ill teenager with deteriorating eyesight that was waltzing around the enemy base as a known double-agent for nearly a decade, who was actively keeping the Big Bad from attacking Konoha again, kill the Big Bad? Because the Big Bad was hiding in the shadows. He was hiding from Itachi so effectively that even the readers didn't know he existed until the 300 chapters in.
Well he "came out of the shadows" after Itachi died so that no one would interfere in his meeting with Sasuke (I believe).