What is the right method to practice origami? Is rote-memorization correct method?
It's because you are still at a stage of learning where you are simply memorizing a series of steps, without really understanding the more nuanced principles of and techniques of origami and how these forms are constructed. 

In neuroscience, it's called 'chunking', and it's a crucial part of becoming proficient at any skill, and it only comes with repetition, study, and practice. With more nuanced learning, instead of memorizing the simple mechanics ("fold this, then this half way, flip over and tuck back"), you'll start thinking in the patterns and techniques that combine a relatively simple variety of folds to make increasingly intricate designs. 

Think of it like cooking. You can follow (or memorize) a recipe and cook a chicken dish that you can prepare over and over without any real understanding or context. But once you learn the more intricate principles and science behind cooking, you start to understand why things are done that way &mdash; so now you are no longer blindly following a series of steps, but following the patterns and techniques you learned to achieve the desired result. Now when it comes time to "memorize" yet another recipe (or come up with a creation of your own), you'll already know about 90% of it before you even begin. That is how you build a repertoire of dozens, sometimes hundreds, of "recipes" without having to memorize each step of each individual creation without context.

The same goes for the techniques of origami.