i got it was because it is supposed to be simple
Ha ha! Does the name Luke Crane mean nothing to you?
No, but seriously: Mouse Guard is kind of in an uncanny valley, complexity-wise. It's loads simpler than recent editions of Dungeons & Dragons (and also simpler than Crane's other game Burning Wheel), but it's also a pretty hefty sucker in comparison to very simple games like, say, Trollbabe or The Pool (the Pool is free and very strange if you've only known "traditional" role-playing games - I encourage everyone to read it!). So whether Mouse Guard is simple kind of depends on what you're used to. I'd guess it's about as simple as Tavis's White Box game once you get the hang of it.
Mouse Guard is an awesome game, though, and I definitely want to play or run it some more.