@Jarod42, I believe it’s not that simple.
Issuing valid S/MIME certificates requires additional trust bits set on root CA certificate and enabling them is on discretion of root CA store operators, such as Mozilla, Microsoft and Apple. This would require Let’s Encrypt to use cross-signed CA to speed up inclusion process (just like with certificates for websites), which would probably require changing their agreement with IdenTrust and/or obtaing separate CA certificate (I guess that IdenTrust is not doing that for free ).
You cannot simply upload CSR to obtain S/MIME certificate, as CA has to perform validation that you are indeed in control of email address. Let’s Encrypt is based on ACME, so I guess that would require changes to ACME protocol specification (and implementing them in Boulder - CA software). It’s difficult to perform email-only validation securely, as validation message may be intercepted between SMTP servers (some servers are not using SSL/TLS at all, so messages are transmitted in clear text).
I guess there may be also some technical challenges, as more certificates would require more OCSP signing capacity - which may in turn require buying additional HSMs (Hardware Security Modules) to be able to sign responses for all certificates. Let’s Encrypt is still non-profit run for the public’s benefit - and unfortunately HSMs are not cheap.