I would guess performance. At least at the time, I believe browsers/http were pretty bad at parallel requests. I suspect Tor exacerbated the issue too.  

Today, image websites (like google image search) still do this for UX reasons - no layout shift and all the images load deterministically. 

k00b

bitcoin

The Underground Website Where You Can Buy Any Drug Imaginable (2011)

"untraceable digital currency".. oh well, that's your average journalist. I had an account on the silk road but never ordered anything. The tech was pretty cool, silk road did stuff like embed images base64 encoded into the html directly so it didn't load external images. Not sure if that was done for performance or security.