Why did they all love Rose?!
She was lame!
I would have been quite happy to watch a film totally about Cassandra and that hunky gardener. Mmmm.
I think it's pretty simple actually, though I did read the book before I saw the movie, so it was easier to interpret everyone's feelings.
*******Spoilers***********
Cassandra was 17, Rose was in her 20s, a much more eligible and conservative age for marrying as far as Simon and Neil were concerned. Neither of them really seriously considered Cassandra as more than a really likable sister-in-law. Even at the end when Simon nearly proposed to Cassandra, they both acknowledged that she was wise not to let it go further. Cassandra states that it's because he's still obviously in love with her sister, but it's also heavily implied that Cassandra is young and not ready to settle down yet - she has more adventures and experiences to live. (Even if Cassandra herself doesn't quite realize that yet.)
Which to me is a good explanation as to why Cassandra wasn't happy to settle down with Stephen, either. He was too much a part of her childhood.
Btw, Simon was much more attractive and likable in the book, and I got the sense that he actually liked Cassandra more than Rose but felt Rose was more conventionally "suitable" to be his wife.
So long, glamour-boy!