In medicine/public health/etc. there are two reasons: 1) medRxiv was relatively new, and while the COVID-19 pandemic was its moment, there are a lot of concern

In medicine/public health/etc. there are two reasons: 1) medRxiv was relatively new, and while the COVID-19 pandemic was its moment, there are a lot of concerns about pre-peer review stuff, and the public, press, etc. not treating it with appropriate caution. 2) Some journals heavily penalize or won't consider preprints, and some of these journals are quite good. This has gotten a little pushback recently as well.