When the Trustees were serving on both the IETF Trust and the IAOC, many of the requirements for naming a particular group of people were driven by the IAOC's requirements. For the IETF Trust in the new model, some of those arrangements were rethought, both in terms of the number and source of the Trustees, as well as the desired qualifications and length of terms.
Several options were possible, of course. A newly designed selection process could have been devised, but in this document we argue for limited change based largely on the fact that a) the IETF Trust arrangements worked generally well, b) the expected time commitment is expected to be modest, and c) the assets need very careful management.
As a result, a smaller group of Trustees appeared sufficient.
In addition, the terms set for the Trustees selected from the IETF community could be longer than the two-year period typical of other IETF bodies.
One could have continued the practice of having the chairs and CEOs from the IETF, IAB, and Internet Society be Trustees as well, but this may not be necessary. In general, the tasks of the IETF Trust are well defined, and while there is a need for coordination, it does not need to be at the level of chairs or CEOs.
Given all this, one approach was to have Trustees appointed by the NomCom, the IESG, and the ISOC Board of Trustees. (One might also have considered the IETF Administration LLC legal entity instead of the Internet Society for this role, but the Internet Society is perhaps more suitable for the role given their focus on the broad use of the IETF Trust assets and not merely administrative aspects.)
If the same principles used for previous appointments continued to be used, then appointments performed by the NomCom would need to be confirmed by another entity. This could be, for instance, either the IESG or the IAB. The IESG had previously been the confirming body for the IAOC, so it has been retained in that role for the Trustees.