Mame roms with several versions also have parent sets. What this means is only one version of the game will have all the roms required for it to run. All the other variants will only contain the few roms that are different from the parent, meaning they need the parent to be in the rom folder to be able to run.
There are 6 different romsets for Mortal Kombat. The parent romname is mk.zip, (incidentally this shows as version 5 not version 1) all the other versions are clones and won't run without it.
Many mame roms are updated over time as better dumps and new versions become available. This means that roms that work with older versions of mame will need to be updated to work with newer versions of mame.
If you have mk.zip in your rom folder and it doesn't work then your rom is outdated and you need to source a more up to date version from somewhere that hosts up to date roms.