The fact that the photos posted in the meta thread have really low scores but the one I uploaded as a post has over a hundred shows that the idea of all photos being in one post doesn't work.
I'd say it's more that new threads attract more views as they're more likely to land in the instance's local feed or c/all. The sorting algorithms suck for megathreads. Still, we could switch to weekly or biweekly threads if everybody is onboard, though I don't know if there will be enough momentum to keep them alive as we're still a relatively small community.