You ask a very good question and I wish had a more deeper answer then I loved them.
Ok if I think harder about it there's a few contributing factors
Nick was already one my favourite characters, pretty sure I hadn't been watching too long when he arrived I remember the storyline with his mum and Irene that was one of his first storyline right?
And I remember Kirsty being down right awful a lot of the time, someone could probably help me out bit more with time line of things but wasn't she with Nick then Seb arrived and she was boarder line cheating on Nick with Seb when Nick and Jade got together? (Did they cross the line when Kirsty was still with him?) And she had big hissy over it even tho she wanted Seb
Then was the Kane stuff in there then remember Kirsty just becoming awful to Seb and dropping out of school then going off my memory she got back with Seb then cheated on him with Kane? So just in contrast Jade and Nick were so much enjoyable compared to Kirsty and her pairings.
But they also gave Jade and Nick challenges to like the bus crash/Nick not walking, the whole Angie saga and again memory foggy but for the most part they stuck together? Just gave you something to get behind and invest in and remember them as some of my first ever favourite couples
Then their brake up was just so stupid and unfair after all of that and insult to injury she gets with Seb after.
I was also young and was easier follow the hype and the feelings of what the show wanted you to. If I watching now age am now might not of been as invested