Because Monero lacks an encrypted memo field. (Monero has encrypted payment IDs but these aren't quite long enough to hold an e-mail.)
Accepting Monero payments requires theshadowbrokers to exchange multiple messages over different channels.
With ZCash, a buyer sends a transaction containing an e-mail address. TSB sends file to e-mail address.
Never fear, Monero developers have been talking about adding a memo field for a while.
Thank you for the explanation.