Just got new tires and an aliment and the front left tire is already bald. why is that?

I just got two new front tires and an aliment not even six months ago and the front right tire is already almost completely bald. About six to eight months before that we got two new tires on the front an they didn't wear down like that at all. When we got the two newest tires we put them in the front and moved the other two to the back an now the front right tire has almost no tread left on it. Was wondering if you could help because i can not afford to keep buying tires left and right. THANKS!