I feel like some fans send such long letters based on program topics so that only their letters will be chosen and read. To stand out. Other letters won't be because of shortage of time.
There's a certain way of writing in, and of course not forgetting continuous effort of sending in, to get read. For personal radios like this one, it's more about finding a connection with the talent of the radio, in this case being Koko-chan. Once she has noticed your name, she is more likely to pay more attention to you. In the case of anime radios, where the writers pick the mails to get read out, they also look out for mails that add value (could be related to the anime itself, or events, or radio segments) and if you write in on a regular basis, they would also keep track of you. I've stopped writing into radio shows generally, but I'd just say writing mails is an art in itself.
