Why is SMTP over SSL between email servers not so popular? [closed]

Good question, I really haven't seen any figures for this. I'm not sure, but I think many large companies now support SSL/TLS for inbound and outbound SMTP ("MX" mail delivery). This is normally optional and can be negotiated via StartTLS on port 25. Most SMTP servers do not require server to server TLS, however, as it would mean many would not be able to receive mail from an MTA that does not support or is not configured for TLS.

Many email clients support TLS between the UA and MTA - either SMTP/IMAP over SSL or POP3 over SSL. I think gmail for example requires SSL/TLS for IMAP and POP3.

Regarding actual end to end email encryption, this is normally achieved using S/MIME or PGP. However, due to the complexities in setting this up and managing it, it has not seen wide-scale adoption.