For two reasons.
First, in spite of the big budget, and the big Benedict, Dr. Strange suffers from all kinds of narrative problems, and gaps in continuity. Mainly, I think, the result of trying to cram too much into a film -- trying to give a typical Marvel backstory while at the same time trying to present an exciting self-contained thriller.
(Dr. Strange also suffers from following too closely the original arcs of the original comics. And, before you start pounding on the NOT USEFUL key, you should know that this reviewer not only read all the originals in the series but actually SUBSCRIBED to Dr. Strange back in the day, which meant that the nice Marvel people actually mailed me each issue as it came out. In an envelope. The original story arcs were far from Marvel's best, compared to its better-known heroes, featuring the same two villains over and over, Nightmare and Dormammu, each taking turns boring the reader to death.)
Which brings us to this ambitious JLA entry, an attempt to show that the mighty DC library can possibly take on the mighty Marvel library, sort of a David Goliath thing....? Frankly, they nail it.
Part of the fun is the script. Man, is it tight. One of the best I have ever encountered in a "superhero" animation. Early in the story, a hot babe who is actually the essence of Constantine's magic house does a 30 second analysis of Batman and concludes he is mainly pain and darkness.
"How do you manage?," she asks sincerely.
"I HAVE A BUTLER" Batman responds deadpan.
That is one of the best lines of dialog I have ever encountered from DC. You could even hear the guys over at Pixar getting nervous. And, even better, the quality of the writing holds up for the entire film. Wow.
To sum up, the high score is because DC/Warner attempted the impossible, taking on a Marvel live action film with their own animated product at a fraction of the cost, and more or less pulled it off. And because the boyz at DC/Warner understood that sometimes less is more, and wisely kept the other (over-exposed) members of the JLA in the background. And, most of all, because this is an entertaining and engaging film.