you know how i interpret someone saying ''plot no jutsu'':
''i didn't understand so instead of re-reading and enjoying the story, ill blame the writer''
Obito's Kamui is obviously not complete, [missing Kakashi's sharingan]
alongside this we can ''ASSUME'' that while inside the dimension, he isn't free to manipulate the phasing as he does outside it
Combine with the fact that his weakness lies in the moment of attack + the fact he has been :
1.Controlling 6 Jinchuuriki and their Bijuus via Rinnegan link
2. Battling the Hachibi + Naruto Kyuubi Mode + Kakashi + Gai
3. Maintaining control of the Gedo Mazo until the summoning of the Juubi
4. Constant use of His sharingan in phasing through attacks [even though he got tagged quite a few times when they figured how it works]
Also Obito is emotionally conflicted no matter how he tries to hide it, [thats the beauty of the story, he keeps trying to justify to Naruto that he is right and the way he used to feel was wrong],
All these add to the reason why he went from an untouchable shinobi, who was hardly ever bothered
to the reckless shinbo who lost to Kakashi
I reckon that he believed Kakashi was not on his level, and he got carried away and lost that tussle