Metal Slug was released in 1995, when CDs were becoming mainstream with the introduction of the Sega Saturn and PSX. CD was considered "the future" and SNK pushed their new cd platform much stronger than home cart. During this time period many home carts were released in limited numbers besides Metal Slug: Stakes Winner, Magical Drop 2, Over Top, Twinkle Star Sprites. To this day these are some of the most difficult games to find for the system. Over time people realized the flaws in the CD based system and there was a gradual return to the Neo Geo home cart. In turn, this return sparked a demand in home carts and resulted in a sharp upturn in prices.
The neo geo's place in video game history is not as an outdated machine, but rather a focused performance machine for 2D gaming. If you want the best 2D on any system you need a neo geo. Just like if you want vector grafics you go with a vectrex.
Anyway, it is unfortunate that Metal Slug was produced during the transition phase from cart to cd.
Carlson