Why The Avengers DVD out in 3d but the bluray is?

You're likely thinking of the original release of "The Avengers" in 2012. It is actually the opposite of what you described:

* The Avengers was released in 3D Blu-ray format. This was a common practice for big-budget action films at the time, as 3D was still gaining popularity.

* There was no standard DVD release of The Avengers in 3D. Standard DVDs, even at the time, lacked the technology to support 3D visuals.

The reason you might not have seen a 3D DVD is simply because it didn't exist. 3D technology was primarily available on Blu-ray discs.