Pelvic cavity is guarded by pelvic bones from lower side. The bladders are lodged here and it finds some free space here to expand and contract. Because bladder is a muscular organ which expands while fills with urine and comes to its normal size during empty stage. The pelvic region gives the bladder such free space for its easy fit.
It is also protected by the hip bones or the pelvic bones. That's why it is okay to place bladder in the pelvic cavity.