The journal appears to be published by an office with 7 FTE's which presumably is funded by the money raised by presence of the paywall and sales of their journals and books. Fully-loaded costs for 7 folks is on the order of $750k/year. https://www.kentstateuniversitypress.com/

Someone has to foot that bill. Open-access publishing implies the authors are paying the cost of publication and its popularity in STEM reflects an availability of money (especially grant funds) to cover those author page charges that is not mirrored in the social sciences and humanities.

Unrelatedly given recent changes in federal funding Johns Hopkins is probably feeling like it could use a little extra cash (losing $800 million in USAID funding, overhead rates potential dropping to existential crisis levels, etc...)