Perhaps because the objective of BEAT is to prevent eating disorders resulting in permanent changes to children's health, to the bodies, to their minds. Successful outcome for BEAT would be the child no longer needing the website because they are able to live a healthy life in their healthy body, freed from the distress of any eating disorder.
Whereas for Mermaids the objective is to support whatever disorder or distress the child has resulting in permanent changes to children's health, to their bodies, to their minds. Successful outcome for Mermaids would be the child being fully immersed in the website's ideology because they are not able to live a healthy life in their healthy body, ending up entirely in the thrall of the distress of any disorder they might have.