there is code for high ground advantage already in the game, you can enable it yourself by modifying the text file game.cfg in the startup folder.
look for a setting called BallisticHeightBonusPercent. here you can also modify other game settings such as market rates, certain particle and ui effects, default map, editor and zoom settings, and what appears to be an unlimited gold mine feature that greatly slowed mining speed after mining a certain amount.
as for why it was disabled, you can read it directly from the devs here, Ask Sandy archive | Remember Ensemble Studios. search for elevation bonus. basically they thought that elevation bonuses favored ranged units over melee units too hard, and was detrimental to balance on certain maps.