>Could you specify?

Nothing in particular, just a general pessimistic view on how things work on the Internet. Simplifying the posting of multiple images is at odds with the need to protect against abusive floods, for example.

In the specific case of using zip files (or something similar, rather than multiple HTML file inputs), you are also now opening yourself to potentially harmful payloads in those files. What if there are a jillion files in that zip? Or something crafted to overflow the buffer of the unzip application? Etc.

It's not impossible to build something to handle multiple-file posting, but hardening it is more work and, for me personally, not much fun. And then, you get to the cultural shifts that might occur, too, as WAHa alluded to.