I would have to say it's that the circumstances are different.
In light and shadow, Edna was a prince who had fallen from grace, displaced by the civil war, only living for survival. Eli too had his own circumstances being a Hayden and carrying the burden of ascending the throne by hook or by crook. In the end, they fell in love and understood each other better through their pasts etc.
Right now, Alex has grown up with a loving family, friends and holds the title of Crown Prince, even. As a protected and sheltered child, he had his streak of arrogance when he was younger, and now he regrets how he cannot sympathise fully with the situation: being poor, hungry, forced to steal for survival.
Jessica, on the other hand, was safe, being the child of a just ruler of the north, and now being shown cruelty by a sick bastard. She now escapes and is trying to live for survival.
I do understand the longing for the familiarity of the older characters, I would definitely love to see more extra chapters, like how many years into the future and all.
Also this is probably Alex's journey around the world of ATLA. He's learning to be more sympathetic to his people instead of being kept to his walls

Reading this story reminds me so much of how I feel about the sequels to both Naruto and Avatar. As much as I would love to read about the new characters, I cannot help myself but crave about more stories/plots regarding the old casts. I just can't seem to move on, making the experience sooo awkward.
I wonder, what makes it impossible for me to relate to these new characters in the same level I related to the old generation?