Because the pay is usually way worse than small animal (like possibly even half the pay...my friend in rural mixed makes less than 60,000/yr and my friends in larger cities get 100,000 or more in small animal), you’re usually expected to be on call even after you put in a full day of work, positions are often in small towns which can have limited opportunities for spouses and children, the work is more physically demanding than other areas of vet med, your clients often don’t truly value what you do and would rather try to handle it themselves and order meds from Valley Vet or wherever rather than have you come out until it’s an absolute train wreck, etc. You have to love it for it to be “worth it” and more and more people are deciding it’s not worth it. Also some of the “shortage” is that the areas simply can’t or won’t support a vet financially despite some people needing the vet care.