It can be due to a couple of things. Multiplayer games tend to often get a very large community behind them as well as clan formation, thus the sheer number of people that actually play a multiplayer game/mod is usually a lot larger than single player games/mods. Also people seem to prefer to play with other people, as pretty much anything is fun with a few people ... as it leads to more excitement of taking someone down and so on. The other possibilities is the single player games/mods are very difficult to make. Not in the technical sense, but rather what is 'fun' sense or the game play factor of the game/mod. Also single player games/mods require an x factor which is hard to describe.
Like take for example Unreal II. A lot of people said it wasn't really worth the money paid for the game, and even though it was a culmination of all other types of SP games out there before, on general people didn't really like it.
Making SP games gives the creators an oppurtunity to tell a story or an event to the player. However if the story/event lacks in sufficient detail then often the player is then detached from the story/event. Say for example the beginning lines are "Year is 2075 and the robots have take over the world". If there is no further explanation about the past events of how this actually occured or further development other than that (the game is about blasting millions of robots into the ground) then the SP game is deemed as another run of the mill FPS.
The second part which is difficult is character development. Because they aren't making a film (thus should restrict in-game movies to a length, e.g MGS2 did way to many) character development must come in a form which doesn't distrub gameplay but also allows the player to get an attachment to the characters within the game. It might be as small as allowing the player to converse randomly with the NPC or in that NPC's slowly reveal themselves to you (e.g by reacting to what you do ... mercy killing makes NPC's more angry, sad, happy). This requires a lot of work, but also requires extra work in the creation of an NPC. For example in Doom 3, there were very little character developments so all of the characters were pretty much forgettable.
Thirdly is the levels. Levels are a very finicky thing. They can't be too long, too short, too straight forward, too complex nor too difficult. You want to achieve the right balance in all of them as well as have the levels connecting to the story in a meaningful way. Unreal II pretty much failed to connect it's levels together to form a story. In Unreal II you went from planet to planet, but it left an overall feeling that the levels weren't really well connected with each other. I believe Deus Ex had a somewhat solid connection between levels.
Lastly (there are many more points btw) there are also other technical areas which often fail a SP game. A good one to point out is how to handle saving. You could either let the player save any time he/she wants but you run into the problem of quick savers (they shoot an enemy, save, shoot another enemy and save) which can lead to the deterioation of game play (they actually complain it is too easy). A possible fix is to have auto skilling of enemies but you can run into dangers there (A godlike enemy on level 3 would be too hard). Or you can only allow players to save at checkpoints. The problem there is that towards the end of most games development cycle those things seem to get longer and longer and ill placed, in that if you died you would have to repeat a lot of repetitive sections that weren't hard but were just long. This is a technical issue more than anything else ... but they are seen in many SP games these days.
All of these issues combine are usually judged on a SP game a lot, and often MP games skip a lot of these issues simply because they don't need to actually do it (Character development can be non-existent in a MP game).
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