Cause Obito's right eye got crushed By a huge BOulder that crushed his skull and squashed His right eye
Only reason ppl think tobi is Obito is because Tobi can move useing space/time jutsu
like Kakashi's MS
another reason they think that tobi uses 4 letters from obito but this might just be a conicedence

someone just kill this theory of obito being tobi