Surely isn't it the case that the unit of a meter is too big for measuring altitude?
Think about it - 1000 feet is about right. Adequate separation, whole unit and we all understand what it means.
The meter, on the other hand, doesn't fit quite so right - 1000 is too big and 100 too small to be useable. Pilots like to keep their numbers simple, right?
Perhaps this is why for vertical measurements thousads of feet has endured, whereas horizontal distances has made greater headyway for metric - viz in meters etc.
Of course there are good mathemetical and navigational reasons why NM will always be currency.