the wikipedia page has links to projects that removed CAcert where reasons are stated. the main one being that CAcert didn't complete a security audit or because they were not yet accepted by mozilla (because of the lack of an audit, but also because CAcert actually withdrew the request to be included). one group removed it because CAcert has a strict root redistribtion license that they can't follow.

LWN has a good writeup on the audit situation as of 2014: https://lwn.net/Articles/590879/