Why would a conference discourage authors to submit their papers to pre-submissions platforms (e.g., arXiv) before submitting them to the conference? I asked some conference organizers whether authors may submit papers to arXiv before submitting to the conference, and got the following reply:


  though not encouraging it, [conference name] allows archiving pre-submissions on platforms such as arXiv: therefore you can upload there your article before submitting it to the conference.


Why would a conference discourage authors to submit their papers to pre-submissions platforms (e.g., arXiv) before submitting them to the conference?



The proceedings of the conference are licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, so 
the license isn't an issue. The only issue I can see is that it may jeopardize the anonymity of the authors (the conference review process is double blinded). Is there any other downside?

I asked the conference organizers why they do not encourage archiving pre-submissions on platforms such as arXiv, but I haven't heard back from them.

This question is different from the question Submitting ACM Conference papers to ArXiv: the latter focuses on licenses, whereas here the license isn't an issue.
Here is a nice article that explains your question from ACL2017 . Main aim is to protect double blind review system.