Fewer games include things like diplomacy, because it is seen as "weak".More likely it is just too much text, which is counted as a negative with many casual and/or illiterate gamers (and increases localization costs).
Also, if diplomacy isn't one of the main focuses of the game, then adding that option is a lot of extra work and game complexity for a path that probably a relatively small minority of players are going to choose.
I recently came across a reference to a freeware adventure game called
Cult...
Using negotiation and trade, one must resolve a situation involving barricaded cultists in a villa two kilometers away from town.
As you play, you wander around the villa, meeting some of the brainwashed cult members (and some reasonable people). You talk to them, find out what they want, and trade items with them for information. You learn a little about the social structure of the cult and even get to save some lives. Advancing from room to room, you find keys to locked doors and operate switches to remove barriers.
