Why bioRxiv instead of arXiv.bio? In view of the need of a preprint repository for biology science, why was it necessary to start bioRxiv, instead of a biology section in arXiv?  Is there any fundamental difference between the two?  If bioRxiv becomes popular now, why weren't biologists willing to use arXiv?
ArXiv was not interested in expanding into biology (apart from quantitative bio). A 2013 Nature news article about the establishment of bioR&chi;iv explains:


  ArXiv’s quantitative biology section, launched in 2003, has seen exponential gains in the past two years [. . .] But arXiv does not plan to expand into other areas of the life sciences, such as cell and molecular biology, and the site is not designed with biologists in mind.


In 2018, arXiv Operations Manager Jim Entwood discussed how the arXiv decides whether to expand into new subjects, saying that they ask "How does the proposed category fit into the rest of arXiv?" and that they are "fairly conservative when it comes to adding new categories". 


  This comes in part from our general principles, and our commitment to those communities we already serve. We also need to ensure sustainable growth both in terms of active engagement with research communities and the financial and organizational capacity of arXiv. Decisions for category expansion are ultimately up to the arXiv Program Director and Scientific Director with input from arXiv staff and the Scientific Advisory Board.