Most traditional wedding suit hire collections are designed using menswear blocks.

These assume particular relationships between shoulder width, chest, waist, hip, rise, thigh, jacket length and sleeve length.
Many female bodies have different proportions. For example, a woman may have a relatively narrow waist but require more room through the bust or hips.

Some differences can be managed through sizing and alterations, while others are much easier to solve when the garment is created specifically around the body.