Because all the shooters who are also smart phone savy will go online and sign up as soon as the match is opened for sign up. No matter if at work or out somewhere, they'll pause long enough to go online with their smart phones and all spots will be taken before some others, not so smart phone savy, can be by a computer to log in and sign up.. some only have desk top computer, and even few may not even have computer, etc...
so if everyone is forced to call in, in slows down the process and more fair for everyone.. not perfect, but more fair...