Yeah, I agree with Tafoid, with one modification.
When a hardware system is no longer being manufactured or is no longer popular in arcades... yadda yadda....
See, once a given system is emulated in mame, it usually is very easy to add another game with the same hardware. MameDev ran into that a lot with NeoGeo games, and don't want a repeat.
For example, a new Golden Tee game came out pretty much each year from 1995 to 2000, but all were on the same hardware. So even though GT '97 & GT 3D (a 1995 game)) weren't being sold in 2000, the hardware still was, so mamedev stayed away from emulating those two.