[Fomite]

Two reasons that often come up:

- Occasionally, society-level journals are being expected to subsidize the society itself, in terms of the annual meeting, student scholarships, etc. They're often also outsourced to commercial publishers, because even fairly substantial societies are run on shoe-string staff.

- If you're NIH, Wellcome Trust or some other organization funded, there's a mandate that the manuscripts become open access after (typically) 12 months. That lessens the pressure somewhat to open up journals.