I'm not really sure. Probably something about tools not respecting or ignoring them or "they don't look as nice" or some other silly reason.
Edit: To clarify, there were a lot of good reasons not to allow them to link to a subdomain, especially they were running marketing software made by other companies, because there is a lot of nasty security issues around cookies with subdomains.
But there was no reason for them not to route their outbound email through a single domain, because the weren't supposed to run 3rd party mailers without deep audits.
Edit: To clarify, there were a lot of good reasons not to allow them to link to a subdomain, especially they were running marketing software made by other companies, because there is a lot of nasty security issues around cookies with subdomains.
But there was no reason for them not to route their outbound email through a single domain, because the weren't supposed to run 3rd party mailers without deep audits.