Either because their personalities don't actually line up, they didn't really seem like they needed to be involved romantically, or they just are straight up not right for each other: Which couple was the worst paired couple, in your opinion, out of the entire FF series?
And why is it Wakka & Lulu?
I'm just kidding
Kinda
And why is it Wakka & Lulu?
I'm just kidding
Kinda
It's definitely more sad if he, as himself, was truly in love with Aerith though - it makes the story that more tragic in my opinion! But I do think Cloud & Tifa make a better pairing in the long run.