Can you help hypothesize why BeliefMap.org is being penalized?

BeliefMap is, on the surface, a very high-quality educational-information website. (E.g. see beliefmap.org/god/exist) But, despite being several years old, traffic does not reflect that quality, at all. And it plummeted after Google's big May "core update" (see image below). I can't express how valuable it would be for me to figure out why. Could you share any opinions/hypotheses? Thanks for helping!