In the beginning they partnered with an existing CA so that they could issue certificates that where chained to roots already trusted by the major browsers.
“Getting a new root trusted and propagated broadly can take 3-6 years. In order to start issuing widely trusted certificates as soon as possible, we partnered with another CA, IdenTrust, which has a number of existing trusted roots. As part of that partnership, an IdenTrust root ‘vouches for’ the certificates that we issue, thus making our certificates trusted.”