It is inappropriate to ask why the Himalayan mountains were formed, as if someone somewhere made that decision. Thanks to plate tectonics, these mountains, like many others, were the result of massive earthly movement.
Earth's outer shell is divided into several plates that glide over a rocky inner layer above the core, a liquid outer and a crystalline inner core. When the rigid plates of the mantle shift because of movement below, volcanos may occur, and other geological events and this is how mountains such as the Himalayas formed.