I suspect this is not really a suitable question for SU, as the answer is subjective.
I'd suggest the following answer - the first spec was defined in 1996 - although Mosaic - the first web browser was released in 1993. Hypertext was really a replacement for simpler protocols which did not display images and text together. In order to be able to display both text and images, it needed/needs to be able to get all the elements of the page, including the images.
The HTTP protocol is also a very good protocol (for what it was designed for), as it was simpler then, for example FTP, and standardised. Because web browsing became so popular it could be almost guaranteed that content could be retrieved using that protocol, ie it made it simple to get the information past firewalls.
It would also have been important for it to have quite a bit of flexibility - you talk about downloading images, but remember that jpegs and PNG's were not even supported initially, and it would probably not have been a leap for the authors to work out newer image formats would be desireable - huge TIFF images (which, from memory were supported) were not really a good fit for a heavily shared 9600 baud connection ( or 14.4k to be generous) - which was not uncommon when the protocol was released.
I do note that the HTTP spec is actually quite simple, in that it defines a header and body. The header describes the type of file which can be arbitrary (using MIME types), so the protocol lends itself to alternative text formats.