Mothers have lifestyles that are exceptionally unappealing and tedious to the child free unless they have round the clock nannies and never see their kids. Miss World is silly like all beauty pageants but they're selling an image and it's not sleepless nights, nappy changes, play dates in the park and theatre trips to see barney live.
They're searching for attractive young women. In their early twenties most women are travelling, collecting their degrees, starting their careers, partying and enjoying their youth. There's nothing remotely interesting or appealing about someone who has so little ambition and curiousity for experience that they neglect to achieve any of that in favour of early breeding, as a result they are no competition to their peers.
They discriminate against mothers for the same reason they discriminate against the facially unfortunate, learning disabled, proudly hairy, mentally ill, dwarves and the clinically obese. They don't fit the package that the organisers are looking for. Discrimination against mothers isn't any worse than discrimination against any of the above.