Question

why did the pandavas leave hastinapura

Answer 1

Because according to the rule of the game which pandavas and the kaurava were playing,the team which will looses the games have to leave hastinapur and had to live in forests.So as pandavas lost the game they had to leave hastinapur and go away.
Hope this helps you.....

Answer 2

pandavas leave Hastinapur because yudisthir had lost the game of chausar with kauravas
so as according to casuality they leave Hastinapur for next 12 years .

Hope it helps you friend .

Have a nice day.