Zzilla - since you asked - it's a question of keeping things clean. If the parents don't clean under the little boy's foreskin, bacteria will build up, and get into the urethra/bladder causing a UTI. Since fever/fussiness/etc. in kids is generally attributed to other types of infections/illnesses/conditions, it can be a while before a UTI is diagnosed/treated - only in severe cases can it actually cause renal failure.
both boys and girls in diapers are succeptible to UTIs - but it's a little easier to clean a circumcised boy or a girl, and there's nothing holding the bacteria as close to the urethra - that's why uncut little boys are more likely to get UTIs.