Wildcard certificates are coming to Let's Encrypt in Jan 2018 (https://letsencrypt.org/2017/07/06/wildcard-certificates-com...).

Also they are not as important with Let's Encrypt given that you can dynamically and instantly provision a certificate for a subdomain.

Without Let's Encrypt getting a certificate for each domain was costly and time consuming. Let's Encrypt solves both problems without using wildcard certificates.