Thinking about it. IR had the most time to become a romantic paring. She was practically living in Ichigo's bedroom and feelings could've developed there easily.
However, the moment Kubo made Rukia push Ichigo to apologize to Orihime after Yami beat her up I knew IchiHime had a chance of happening.In a sense Rukia supported Orihime and most likely knew of her crush. I'll admit I got very scared when Orihime voiced her insecurities about Rukia and Ichigo's close relationship but it made me respect Orihime so much and I fell for her character more because in a way it reminded me of Sakura's own insecurities. I figure this moment for IR fans made them believe the IR was going to happen as well.
Allow me to add :
When Inoue goes with Ulqioura to Hueco Mundo everyone suspect her become a traitor and goes with the enemy on her own will, guess only one who still believe in her??