Well, duh!
First of all, to all you people squealing about the downfall of western civilization when TMNT overtook Guardians on its opening week - of course it did. Guardians was in its second week, TMNT was fresh. If you were to compare Guardians opening week (90 mil) and TMNT opening week (60 mil), you get a much clearer picture. Also, TMNT dropped off dramatically, while Guardians are rolling steadily along for several weeks now, as the word keeps spreading about what a great movie it is, and people are even going for second viewings, while all that's spreading about TMNT is its own stench.
As for why Guardians are so successful, that's also easy - it's good, it's fun, and it's pretty. Rarely is a movie all three of these, with most summer blockbusters being fun and pretty, but not very good. Add to this that its setting and genre isn't exactly overflowing at the moment (not many space opera type films flying around, is there?), it's a breath of fresh air. Funny enough, when I try to recall the last movie that had all three of these, I land on another Marvel movie, Winter Soldier, which also did VERY well at the box office. Funny how that works out, isn't it? Almost like people want something worth seeing before they'll go to a theatre...