[simcop2387]

I'd bet part of it was making sure that it wouldn't leak information. They've spent a good deal of time on that in the past few years (images in email are fetched by google, rather than your browser). If done incorrectly it could be used to help fingerprint the email client in some way (screen size, which queries are listened to, etc.) which could enable another email to be sent that attacks a vulnerable device (think stagefright like media attack).