My father got cdiff while in the hosp, why don't drs prescribe probiotics more frequent to increase the good flora when on strong antibiotics?

Unproven. While some studies show possible benefit from probiotics more studies show an inconclusive benefit. It appears that active antibiotic use destroys the helpful bacteria in probiotics making them useless. In rare cases, probiotics have become unfriendly and caused unusual infections in people with weak immune systems. Probiotics may have a more useful role after antibiotic are stopped.
Doesnt always work. Your dad could have gotten c diff even if he was already on probiotics. Also, some probiotics are killed by the antibiotics too. Obviously, being in a hospital is a risk factor but there is likely an immune system issue in patients who develop recurrent c diff. Unfortunately, c diff is here to stay and prevention is more important then cure.
Good question. Most drs either are not keen to think about 2ndary causes of antibiotic side effects to the gut flora or too involved in treating the infection. The benefits of using probiotics has surged and proven to be effective. I agree that this is overlooked when prescribing antibiotics particularly in elderly patients.