It's because of how many mailing lists don't really work with DKIM
Basically, Google's enforcing DKIM from certain domains, and if a message is "from" someone whose e-mail host provides proper DKIM, but it's missing it, Google (and Yahoo) servers reject it. Mailing lists aren't usually set up to properly handle DKIM (being, effectively, a relay), and therefore get rejected.
The workaround that I saw one mailing list use was to resend the e-mail from the mailing list's address, append "via (mailing list name)" to the name on the from field, and just have both the mailing list and the original author in reply-to.