I bought a Swiss Military by Chrono SM34004 quartz analogue watch. Its battery died after only 6 months. Why did that happen?

Old stock. 

The battery was already old when you bought the watch. Or a very cheap low performance battery was fitted.

The other possibility is that the watch mechanism was a bit stiff to begin with, and that puts a lot more load on the old battery. If the next battery fails prematurely then the watch is definitely faulty and needs a repair or replacement under warranty.

That is actually quite common.   

Cell life varies widely, too.   

Many quartz watches are shipped running, 

which depletes the cell. 

My Seiko 5H23-8A09 was made a year and a half before I bought it 

and was running when I first saw it in the jewellery store. 

I can't remember how long the cell lasted after that, 

but it was OK for a long time.