Because making a MP game requires creating or using an existing engine, then optionally tweaking for the purpose of the game, then adding some assets (skins, sound, etc. etc). There would obviously be a little more more coding involved but that's about it.
Making a SP game involves the above but A LOT more content/assets and development. You have to get writers involved, way more man hours of developers, sound engineering team, the graphics folks, (more) voice acting, m...
That analysis is a bit lacking and more like a play-by-play description of what happens in the trailer.
1. There's a Repear-like sound in the background when those structures rise out of the ground.
2. One of your squad mates running with you into battle is KKRRROOAAGGAAAN!
Personally, I feel like the music ruined the trailer for me -- Johnny Cash when you're expecting some original Mass Effect themed music or least some kick-ass Two ...
Considering SP development requires a lot more time/money/effort than MP, I'd say even $40 is way too high a price for an MP-only game. $30 at most is being generous.