Why do the authors think it's a good idea to shift the MC's main focus to someone other th...

Why do the authors think it's a good idea to shift the MC's main focus to someone other than the main character? I already love the current MC, so putting someone else in his place will only piss me off. Especially when Ian already has imposter syndrome and everyone treats him like Brian. And then the author changes the narrative and boom we have a story about Bran and Sol.
Just make a separate story about them and focus on Ian here pls. Or give Ian a third love interest and let him be with someone who sees him as himself.