How do you get your employer to let you WFH?

Id be interested as to why they have refused a flexible work request. I also work in insurance but we're all hybrid, 60% in the office.
The only people who aren't allowed to WFH are new hires who have to complete 3 months in office in the beginning and then they can WFH if approved

I work in a completely different field but I’ve already discussed WFH later on in my pregnancy as this is something that is allowed in some departments routinely (not mine) my boss has said as my risk assessment has to be reviewed again at the start of my 3rd trimester then we will discuss it then but he’s not against it especially as I didn’t choose the work location I was given, my commute is an hour each way & I work 10 hour days so fatigue will be a factor and our office gets hideously hot in the summer months.
I think many businesses are now trying to move away from WFH but if you can find genuine reason to support a request I’d maybe see if you have any support networks in your company that might be able to assist you in a further request.