Tidus and Rikku are tempramentally similar and compatible. Both are cheerful, sunny, and somewhat goofy. But to bring them together as an item would require heavy rewriting of the script.

Had Tidus stayed with Rikku on the salvage ship and not been dropped off at Besaid, they might have had time to deepen their friendship. But by the time Rikku reappeared in his life, he had met Yuna. Any feelings that Rikku may have had, she kept to herself.

Yuna is Tidus' true love and no affection rating games can change that outcome. I wish it had been a little less preordained, but you can't have everything.