Generally we attribute a greater sense of civil government functionality to the people who actually fought to create it for us. In this context, these folks believed that for an engaged citizenry and a government reflective of their values and lifestyle, an apportionment of 1 per 30k voters was functionally appropriate. Whether it is correct, I don't specifically know - but, I sure would like to find out. ;)

The original text provides a bound not an equality: "The number of Representatives shall not exceed one for every thirty Thousand...".

They didn't want too much direct representation but apparently didn't worry about too little. That said, I have never dug into the history and I am taking the text at face value.