Why all company HR's asking same ques ?

i have completed electrical engg at 2008. after completed my degree , i had joined in electrical contractor company... but now i wish to change my career to production line,,, but HR asked one ques wherever attend interview.. why do you like to change your field or company asked by company HR. i wouldn't like to continue that job due to timing of work... daily i leaving at night 9 pm or 12 pm ... its reason to change the company, but how can say this reason.... if i say mean they mean it at wrong like i'm not interest in work... how i will reply for that ques.... plz help me by answer my ques...