I don't think 90% of games have online multiplayer. Many games are just not designed for that.
In terms of development, multiplayer is generally much more difficult to deal with, although there are also platforms that do some/all of the dirty work for you so that can help. The scope of the game obviously makes a big difference too. The advantage sometimes is that you have emergent gameplay coming from players actions, meaning you might have less work to do in terms of creating interesting gameplay (you just have to make the mechanics and then let players go to town). Then again, you might end up programming bots anyway to help with empty servers :P
Viewing post in witch is Better PVP or PVE
many games nowadays came with the idea of multiplayer and if it is pure Singleplayer it must have good story line with game design to work either way it might die quickly but multiplayer have a longer life time. now since adding multiplayer late in the development is a headache usually developers decide even before starting development making having your game SinglePlayer and multiplayer is the best but it's it really the best option?