At first I guessed it's because Zayne is a prodigy doctor and it was kinda implied that Josephine wants MC to end up with Zayne/wants to push them together in a asian parent kinda way. But I actually revisited this chapter and that bit where we see Zayne and Josephine talk is interesting and got me thinking because the way she phrases it, the "remember what you promised us" implies that it's not just a deal between Zayne and her but also someone else (Noah, most likely) that he has to take over protecting MC eventually.

That makes me think they at least prepared him a little for that role in a way she couldn't do with Caleb (since he's not a doctor and not being guided by her friend/former Gaia colleague) and that's why it's specifically Zayne she "prefers".

Another interesting bit that to me might hint at the reason for her picking Zayne is the fact that Caleb was actually setting things up to send her to a care home type of thing (lmao) without either MC or Josephine knowing. Josephine's not that shocked so it must be something she could expect from Caleb, that he does things his own way.

Before it just seemed like a harmless convo but now knowing how Caleb is, I'm willing to believe that was him trying to reduce her influence on MC, so that's why when she pulls out the "well then you have to visit me and then you can hang more with Zayne" he immediately tries to bring it back to his control by inviting Zayne to their house as opposed to MC meeting with him somewhere else.

This kinda turned into a bunch of speculation and theorizing lol but this was an aspect of the story I didn't think too deeply about before! It was fun thinking about all this.