Why the DC animation much better than Marvel?

DC animation is generally better as they have Warner Bros. best animators while Marvel animation doesn't have Disney's best animators.

The DC animated movies and TV series is made by Warner Bros. Animation (alternatively as Warner Animation Group for theatrically released films), the animation division of Warner Bros. which has produced critically acclaimed works such as The Iron Giant & The Lego Movie. Hence Warner Brothers has their best animation team making DC animation.

The Marvel animated shows are made by Marvel Animation a subsidiary of Marvel Entertainment which also does Marvel Comics and Marvel Television (e.g. Inhumans, Iron Fist). Similar to Marvel Television, Marvel Animation generally co-produces with another studio resulting in varying levels of quality (e.g. Avengers: Earth's Mightiest Heroes was with Film Roman while Avengers Assemble was with Man of Action). Disney's best animation studios Pixar Animation and Walt Disney Animation Studios are focused on movies. Disney's best TV animation studio is arguably Disney Television Animation (which has produced Gargoyles, Kim Possible, Gravity Falls, Phineas & Ferb, Sofia the First, Elena of Avalor). In my opinion, Disney doesn't have their best animation teams doing their Marvel Animation.

When Disney finally let Walt Disney Animation do an obscure Marvel property we got the Oscar-winning blockbuster Big Hero Six.

The animation studio is much more important than the comic source. Just look what Pixar was able to do with their Fantastic Four inspired Incredibles compared to Fox's adaptation.