There are a number of situations in the 5471 instructions that call for the attachment of a statement, a company org chart, or other items. Turbotax seems to have made a policy decision not to allow any PDF attachments, even when permitted by the e-file format. So my guess would be that a lot of people who need form 5471 wouldn't be able to use it even if they included it, because of the blocking of attachments. That doesn't help you, of course, but I think that could be the reason.
