Huge initial fixed costs and fairly high operational costs as well.

They have to go to vet school for 4 years, plus they need some undergrad as well. Usually 2-4 years from what I've seen. So they could well have huge student loans. Moreso if they specialize to become, e.g., a veterinary surgeon.

To have their own practice, they have to set up a clinic. Building, surgical instruments, X-ray machines, incubators, heart monitors, anaesthesia equipment, ultrasound machine, dental machine, computers with special veterinary software to store/view information/scans, great big stainless tables, kennels, scales etc. All of this has to be maintained and occasionally upgraded/replaced.

Continuing Education. Good vets don't end their education after university. They have to keep up to date on more than one species, which might mean subscriptions to costly veterinary journals, going to veterinary conventions all over the country, going to special lectures and demos by specialists etc.

Cost of staff time: the veterinarian might spend hours researching the animal's specific condition and hours more doing the surgery, and more time still to check up on the animal afterwards; vet techs to do routine care, help with basic procedures and monitor during surgery; vet assistants to answer the phone and help clients; people to clean the kennels and rest of clinic, etc.

Cost of anaesthesia, any surgical equipment used, IV, injections to jump-start their digestive system or to boost some other function, antibiotics, special medications, painkillers, food, supplies used to do in-clinic tests, etc

Cost of sending off speciality tests, bloodwork, biopsies etc. to laboratories.

The dog in question required research to diagnose, special diagnostic tests, special pancreatic enzymes, a few months of boarding and a (IIRC) fairly lengthy surgery.