Of course he’d bond only with Twilight. He’s her slave.

Good

Except in FIM, the main focus is for the ‘camera’ to show us Twilight and her friends stories, how they interact with each other and the world around them. If it happens to feature Spike, it did so either because it featured an event significant enough to impact his character development, or how he is perceived and interacted with by Twilight and her friends.
 
Also, Spike often appears either as a slapstick character or the fall guy. Maybe he didn’t appear because they couldn’t include Spike without breaking the viewer’s immersion in the story being shown.
 
It could also simply be that it wasn’t something which the studios/Hasbro could/wanted to do, given that with the amount of characters in the episode, it would have been too challenging/impossible to include peripheral characters like Spike. It could be something else, like budget considerations or the voice actress not being available for that episode.

Let’s just see how season 9 (the final season) works things out for him.

@Background Pony #106E  
they did seem to bond in Celestial Advice

@neo4812  
Talking about him and Tia  
Twilight’s parents tho as well

@Background Pony #106E  
spike and twi have bonded loads of times or are you talking about tia or twi’s parents?

@neo4812  
Doesn’t really make much sense cause we don’t really see the two of them bonding that much either

I mean maybe celestia raised him, who raised spike will probably remain a mystery