The Tor Browser Bundle uses the certificate authority list that is created and maintained by the Mozilla Foundation (aka the default certs that come with Firefox). Mozilla has a strict policy for what can and cannot be included in their list, and a large developer base that will hear about breaches, attacks, etc. and can respond fairly quickly to revoke a certificate or add new ones that meet their criteria. There are many, many CA's, and not that many Tor developers; it makes sense to use the existing certificates that are maintained by a much larger (and more experienced in this field) community. This is (hopefully) a safter, more secure route for everyone.
The inclusion status of CACert in Firefox (and therefore in the TBB) can be found on Mozilla's bug tracker as Bug 215243. It is essentially awaiting an independant audit and a resubmit from the CACert team.