There's already a thread about this: https://community.letsencrypt.org/t/please-support-wildcard-certificates/258
I will add my 2 cents though.
I can see the value of wildcard certificates in a world of manual certificate issuance and maintenance. But with automation, wildcard certificates are somewhat of a liability and a complexity. (I know from experience since I'm writing a web server that automatically issues and renews certificates for all the sites it serves.)
Wildcards can be a liability because if an automated transactions fails, malfunctions, or is compromised, all the subdomains go down with it.
From a complexity perspective, it is easier to match domains to certs 1:1, rather than ∞:1 which requires extra programming logic.
Any security implications aside, I would not be opposed to wildcards, but I can understand why these are not (yet?) available.