Why are some people more suseceptible to uti's (urinary tract infections) than other people?

Depends : Infections of any kind are caused for a variety of reasons. Uti's are no different. Getting a UTI depends on the type of bacteria, the strength of the immune system (immunocompromised people are much more prone to all infections), gender ( females are more susceptible to uti), and underlying medical or urologic conditions ( ex, diabetes, stones).

Hygiene and genetics: Some patients have cells lining their urinary tract that allow certain bacteria with p-pili (an example of a type of bacterial attachment arm) to enter their urinary tract and cause infections of the bladder and especially in this example infections targeting the kidneys called pyelonephritis. In addition, differences in hygiene, and other comorbidties such as diabetes predispose to infection.