[cmorrisrsg]

Copying this from my blog comment answering the same question...

Pseudo-transactional campaigns are very "pseudo". MailChimp campaigns require static predefined content. If you actually wanted to vary the content from recipient to recipient, you need to find a way to stuff the dynamic content into merge fields as you send which can cause latency and concurrency issues.

Also, since pseudo-transactional campaigns uses our sending infrastructure tuned for large bulk mailing, the delivery times are somewhat longer (on the order of minutes) for pseudo-transactional campaigns, vs seconds for STS.