Why are bacteria in hospitals often more resistant to antibiotics and more likely to cause sepsis?

Selection/evolution. People bring all kind of bacteria to the hospital. Powerful antibiotics are used, so only resistant bacteria survive and lurk around. Once they find a victim, they jump in. Furthermore, people in a hospital have in general a weaker immune system.
Antibiotics. Patients in the hospital are generally sicker than those who are not hospitalized. Often, they bring in with them more resistant strains of bacteria as well as more exotic and dangerous strains of bacteria. This great concentration of pathogens are capable of having emerging resistance to the more common antibiotics. This, in turn, leads to more serious infections (i.e. Sepsis).