Full Idea
Von Neumann's decision was to start with the ordinals and to treat cardinals as a special sort of ordinal.
Gist of Idea
Von Neumann treated cardinals as a special sort of ordinal
Source
report of John von Neumann (On the Introduction of Transfinite Numbers [1923]) by William D. Hart - The Evolution of Logic 3
Book Reference
Hart,W.D.: 'The Evolution of Logic' [CUP 2010], p.73
A Reaction
[see Hart 73-74 for an explication of this]