A: I don't understand why they did this. It makes no sense.
B: Companies don't do things for no reason.
A: What is the reason then?
B: I don't know - most likely they did some sort of calculation.
FYI, this type of exchange is information-free.As I learned long ago, don't expect venture-backed and post-IPO startups to act "logically". It's all a big hustle and a scramble for market share and whatever other metrics. There's a lot of collateral damage along the way, employees and customers be damned.
Startups aren't a "meritocracy" and management teams don't operate by best ideas win. Markets are brutal, management teams are opportunistic and often self-serving, investors are focused on growth at all costs and hitting metrics, and companies don't operate in ways that suit logic. They suit the needs to capture markets or do whatever they do to get ahead. Sometimes ethically, sometimes not.
C: The calculation that caused this was probably some black-box political battle that was won by Manager A, and lost by Manager B. It was probably decided based on personal preferences and biases of decisionmakers, as well as on who was friends with whom. Decisionmakers that run private corporations make these kinds of decisions all the time, and any attempts at reading the tea leaves, or the chicken entrails, to try to predict or explain these decisions is likely a waste of your time. If you'd like these decisions to be made in the open, you need democracy and transparency, which is not something that's is selected for in the modern business world. This is not a value judgement of the modern business world, it is just a descriptive statement about it.
It doesn't necessarily mean it couldn't have survived on its own if it were spun off. It just may not be what the decision makers want to be thinking about right now. It's not always about what's important to the company (i.e. the company's money), sometimes it's about what's important to the CEO (who really wants to think about other things, and if they have a major distraction that maybe doesn't go well it could add to board pressure on him to step down).
And sometimes (often?) that reason is "I'll show him who's boss around here, don't care if it costs millions of dollars and tanks the stock price, I'm untouchable and they're not gonna forget it!"
Of course, this is just pure speculation.
Just because you or I don't see it does not mean it does not exist. The owners are probably much better informed that we are.
More likely explanation is they did some kind of calculation and decided it is not worth it and very likely we will not get to know it.