Why do we call "Kiwi" or "Penguin" a bird though they can't fly?

Even they can't fly but they still having small wings.. Plus they also having feathers like other birds. And as far I know they are not only birds which can't fly.. There must be others too.

For the same reason as you do call hen and peacock a bird although these two can fly for few feet or what about ostrich , what would you call it?

Bats have wings but it is a mammal not a bird.
Kiwi, Ostrich and Penguine belong to bird category. Their food habits, internal organs and outer body makes them bird. Moreover they lay eggs. This makes them different from others.
There are some insects and animals who lay eggs but they don't have habits or quality of a bird.

Kiwi is called a bird though it can't fly, because they have few characteristics or organs similar to birds.

Kiwis are called "Flightless Birds". It means the inability of Kiwis is an inborn characteristic. Kiwis are not the only Flightless Birds. Ostrich, Emu to name a few too cannot fly though they are birds.

 

Since they are flightless birds.