It's usually a costs vs. benefits decision.

Costs:

• Create your own CA infrastructure or buy a public certificate for each sender
• Teach employees how to use it
• Teach employees how not to use it, especially how to make sure that the secret key is really kept secret
• Teach the customers what this strange stuff in the mail means
• Properly deal with certificate expiration, revocation and all this stuff
• ...

Benefits:

• Usually the argumentation goes like this: nobody else is using this so there cannot be lots of benefits

Thus unless the benefits are higher than the costs or some regulations require the use of signed mails it will not be implemented.

It's usually a costs vs. benefits decision.

Costs:

• Create your own CA infrastructure or buy a public certificate for each sender
• Teach employees how to use it
• Teach employees how not to use it, especially how to make sure that the secret key is really kept secret
• Teach the customers what this strange stuff in the mail means
• Properly deal with certificate expiration, revocation and all this stuff
• ...

Benefits:

• Usually the argumentation goes like this: nobody else is using this so there cannot be lots of benefits

Thus unless the benefits are higher than the costs or some regulations require the use of signed mails it will not be implemented.

Apart from that correctly using S/MIME is not that simple for the recipient too. While there might be indicators which show if a mail is signed or not few understand how these indicators look like, what kind of different indicators there are and that you should not trust any indicators which are included in the mail itself and try make the user believe that everything is secure: i.e. something like trust seals, "scanned by whatever antivirus" messages etc. Thus there is also the cost of teaching all the users.