Some airlines just don't like them and / or pay for a premium service.
Sometimes they work fine, others times check in issues, baggage, visas etc. require agents.
Tui, with 90% Uk or European passports can use kiosks as there are no major issues (maybe not Melbourne in Florida due to Estas etc. when they use desks only)
BA with many nationalities going to places with restrictions are a different kettle of fish.

I've seen Lufthansa for example requiring you to go through up to 15 pages of entering details to check in Vs. 30 secs or so on a desk, or 5 mins if you know roughly what you are doing on a kiosk.
Other times, a mobile BP scan prints a bag tag in seconds.

Menzies pay, or are paid for minimal agents anyway. At least one extra would be required to watch the kiosks on top of 2 minimum at the desks.

Said before, Aer Lingus for example, use kiosks, but after tagging bags, half the time you end up in the same queue as everyone else and still need some doc checks.

Gulf at present have about 6 desks. Not enough for the 2 business desks they want, say 3-4 online check in, and 2-3 for economy. It means that if you check in at home, and just want to drop a bag off, you're in the same queue as 170 people off to Islamabad or somewhere with trollies stacked with overweight bags and on 8 different bookings amongst their 10 family members who all want to sit together on a full flight and then request 4 wheelchairs at the end