I thought you said you understood. BM worked with BE as part of the ME team as a whole. They are under the same company umbrella. It makes no difference if they are in a separate building.
BM created content for the same game. They didn't make another game altogether. Unlike the next ME game. No matter how you want to spin it. The core issue remains the same. The efforts put into MP lessens the potential content for the SP.
Its not really that simple games can have both a great SP and a great MP, there are plenty of games that have done both SP and MP very well. EA is the one who provides the resources for game development, they gave them extra resources to allow development of it if there was no MP the there would be no extra resources, they didn't take anything from the SP.
http://askagamedev.tumblr.com/
You can ask this game dev about how MP development affects SP development don't expect an instant answer since it will require a bit of reading.