Because the messenger boys who started out in the job didn’t deliver.
it’s fine to speak about diversity and equality. The reality is that different jobs require different skills and talents. The initial effort to put messenger boys and young telagraphers in operator positions failed because in those positions were different than the messenger or key pounding that existed heretofore. Running a message required the ability to get an address quickly and a certain degree of spunk to get ahead. Read Horatio Alger. Being a key pounder was a highly technical skill that required getting things done as efficiently as possible. Thomas Edison started as a key pounder.
Being a telephone operator required the ability to communicate with people and get the information necessary to make the connection without putting the customer off. In the early days of the telephone this was important as the businessmen who were the early users of the phone were paying significant money for a service that wasn’t very broad. So putting women I the job made the customers more comfortable and the polite businessmen customers would be less likely to get into pointless arguments with an operator over a connection. So making women phone operators worked for the early phone companies.