Because 'The West' is making the argument this is a matter of legality and due process - that people have commit crimes and must face consequences irrespective of the political situation.
We perceive China as having a politicised Judicial system where people are arrested charged and punished often based upon their feudal status, not objective facts.
China probably has a more cynical view of the West, which is fair, because no justice system is perfect.
If the US released her on the basis of some kind of 'Trade Deal' then it completely wipes out the credibility of the justice system in the US (and therefore Canada) and demonstrates that she was held on what were political charges not real crimes.
Without saying anything about politics, I basically hate Trump, but ironically the only thing he's doing close to 'right' are his actions on China trade. That said, I don't think he cares a single bit about the credibility of Western institutions, and if he can, he will use her as a negotiating pawn in a heartbeat. I think Trump is so arrogant and lacking in self-awareness, that he doesn't really even understand the concept of 'objective justice'. I think he really sees the world in terms of 'making deals' and apart from that 'who cares / what's the point'. The obvious opportunity to user her as pawn makes perfect sense in his mind, I think.
I means the next time Canada has to arrest someone on behalf of the United States, and has to tell China that 'we have a legal process' - it will have zero meaning or value. It will 100% justify China's actions in grabbing diplomats or whoever off the street as a tit-for-tat.