The ESRB system was voluntary, though all of the console manufacturers and certain retailers would eventually come to require the ratings.*
Considering that SNK was both the primary console manufacturer and game publisher for the system, in addition to being primarily concerned with the japanese market first, it’s understandable that compliance with the new ESRB rating system would have remained a lower priority in 1994 (lack of market share in the US notwithstanding). As noted by an above poster, the Neo Geo Pocket Color games would eventually be sold with the ratings on US store shelves circa 1999.
The japanese CERO rating system wouldn’t debut until around 2002 so it makes sense that the japanese home system, Neo Geo CD, Neo Geo Pocket, and Neo Geo Pocket Color titles would lack a retail CERO rating as well.
*sources:
Frequently Asked Questions are posted here. If you still have some other questions, we would like to answer them for you.
www.esrb.org
CEROのRating Systemです
www.cero.gr.jp
**obligatory:
ESRGoob