MP receiving 10 times more fixes than SP in every single update is disgusting, given the bugs still present in campaign mode to this day. the Hanar quest, the Omega glitches (embarrassing cutscenes AND Shepard getting stuck in the door bypass animation loop), getting stuck on the Normandy bridge, the audio cut-off glitch.......list goes on and on.
The reasons are really quite simple when you think about it. It's a matter of making effective use of resources.
First of all, it's a matter of how often players encounter these bugs. If there's a glitch in a single player cutscene, the player only encounters it once per playthrough. Once every 40 or 50 hours or so. In contrast, mutliplayer mathces are quick. If a player puts the same amount of time into multiplayer, they could easily encounter a single bug dozens of times. Players will encounter them far more often, and thus they take priority.
It's also a question of the game looking good vs. being functional. A minor visual bug is annoying and distracting, but really not a huge deal. The game goes on. Whereas a bug in gameplay can be very serious and even break the game. Having the game work correctly takes priority over the game looking perfect. Multiplayer is going to have gameplay bugs and not cinematic bugs for the simple reason that multiplayer has no cinematics. If gameplay is broken in multiplayer, the entire game is broken.
It also needs to be considered that statistically, many players only play the campaign a single time. And often not even that. In contrast, multiplayer tends to retain a large number of dedicated players for much longer periods. Dedicating effort to fixing bugs in single player is going to be a wasted effort for many players simply because they'll never see the work you put in.
Finally, single player has a huge variety of locations, characters, animations, cutscenes, scripted encounters, dialogue and so forth. That is a massive amount of content to test for and fix bugs. In comparison, the amount of content for multiplayer is reletively small - maps, weapons, and enemies - but no cutscenes, cinematics, characters, and scripted encounters. Therefore it's going to take a lot less work to find and fix bugs in multiplayer and single player.