1) Triggering the fingerprint sensor requires an active action, 'touch this sensor', versus Face ID (even with attention-detection enabled). This is important for law-enforcement. It is easier for law enforcement to get you to look at something that it is to compel you to unlock something.
See for instance https://www.wired.com/story/police-unlock-iphone-face-id-leg... Note that some courts have ruled that Face ID should have similar protections as Touch ID, but the issue is far from settled and is likely to end up in court in the future.
2) It's easier in mass-transit and POS scenarios to tap your phone while touching the fingerprint sensor. The flow for Face ID is much more convoluted -- bring the phone near the reader (which may be away from your face) to bring up the payment UI, then bring your phone to your face if the reader is faced away, double-click the home button...
3) During meetings, it's easy to sneak a glance at notifications (if your notifications are set to show only when authenticated) by touching your fingerprint sensor -- there's no need to bring your phone to your face and make it obvious you're checking your phone.
I don't mind Face ID, to be clear. But I always felt Touch ID was more unobtrusive.