A vocation to the priesthood only exists if a bishop accepts a man for ordination. Even if one completes the seminary, until one is ordained, the vocation does not exist.
This creates great uncertainty. The years spent in seminary are a period of discernment for both the seminarian and the bishop. It is a considerable expense for the diocese to support a man for years, with no guarantee that he will be accepted for ordination at the end of the process.
Because of this risk, vocation directors must very carefully evaluate candidates. They will look at physical and mental health, debt and financial history, family obligations, past immoral behavior, criminal convictions, and numerous other factors that speak to a candidate’s fitness to serve.
A diocese may reject a candidate for any one or more of these issues.
The priesthood is a life time commitment, that can be lonely and physically arduous - traveling to multiple worship sites, early morning services, late night vigils. A man must freely accept this burden, and be free of external conflicts, such as caring for I’ll parents or disabled siblings. Very few men are capable of the challenges, while also living as an honest example of Christian piety.