Answer:
Because It is hard for the police to carry out the tools necessary for the measurement out side the police station
Step-by-step explanation:
Bertillonage is the record keeping process that conducted by the police to record several important data about the criminals ( including their size, arm length span, scars on their body , distinguishing features, etc)
The purpose of this record is to alert them in case the criminals that they let go repeat the same crimes again in the future. They need a lot of tools to properly measure this data, and they cannot do it unless they have them in their custody.