(Heya, Roxas here! Now there are a TON of Creddie and Seddie shippers on here, and a few times I've been asked why I don't ship Creddie or Seddie. It's a legit question, so I'll answer.
Were I a casual fan, it's most likely that I would ship Creddie. Statistically, casual fans do ship Creddie, because that's the first impression one can see. At a glance, it's easy to note that Freddie has/had feelings for Carly, and they had a dating storyline in iSaved Your Life.
So, why don't I ship it then?
Shipping always seems to exclude Freddie, but I think Freddie would be better suited for Melanie, who's sweet and accepted him for who he was, no questions asked. Just because I ship a couple that excludes Freddie doesn't mean I don't consider him. I feel he is better suited with someone who is not Carly or Sam. Consider that Carly's told him no already, and Sam, well she hits him and picks on him. The whole world knows that Sam's different with Carly, her behavior wouldn't fly with Carly. iChristmas shows that she'd be worse without Carly as a best friend.
This is not to say that Freddie's perfect. While I don't approve of the iMeet Fred moment, I believe that was a major writing flaw, by two director's who I feel don't grasp the Sam character correctly. Sam is not a vicious monster, and she has shown a nice side on rare occasions. Freddie, on his account, I'd say remembers her meanness too well. He's got her figured out, so when she is nice, it throws him for a loop. It's not his fault, but whoever decided to make the knife joke in iSell Penny Tees also failed to note Freddie's character. He's a nice guy, not perfect, but a nice guy.
For the small few times Sam has shown this nice side, sometimes Freddie rebuffs this. In iReunite With Missy, she asks for his help, only for him to accuse her of being jealous. He helped her set up a website in iDate a Bad Boy, and she became too demanding of him. As soon as she asked for more help, he snapped at her. Knowing this, it could also reason with why Sam wouldn't admit a crush to him. At times, Freddie can be prideful. She's knuckled under, and this is seen in iWill Date Freddie and iTwins. So, no, Sam's behavior isn't great, but Freddie is no saint. None of them are, actually, and I like this.
The Carly factor is great, because she's a normal, popular girl, but so much more. The fact that she dated Freddie shows that she is not the attention-getter that iSpeed Date implies. She cares about Freddie, though I'm not fond of her teasing him, or her jealous streak in the extended iSaved Your Life. If anything, the major Creddie episode to me was more iKiss, than iSaved Your Life. iSaved Your Life, could've been a resolution to the question lingering from iKiss: Why didn't Freddie have his first kiss already? Carly has her fair share of flaws as well, which automatically eliminate her form being a Mary Sue. She digs hot guys, iDate a Bad Boy showed she could be shallow, and naive to relationships. Arguably, iDate a Bad Boy, iStart a Fanwar, and iParty With Victorious note how quickly she gets over people, who she claim to be her "future husband", or guys she "loves". They all come and go, but who remains constant? Sam and Freddie.
Much like Creddie, Sam and Carly are an amicable couple. Sam wouldn't dare hit Carly, and for the most part, she treats her with respect. Yet, like Seddie, there's bound to be surprises, upsets, bumps in the road. A girl/girl relationship isn't easy, and combining the prissy attitude of Carly, with the risk-taking Sam's, something would have to give. This is proven from the episodes iDon't Wanna Fight, and iQuit iCarly.