why are old fashions still trendy

What is the allure of a 1930's trench coat?  Why do we make sleeveless vests with pockets and cargo pants even today?
Fashions change, unfortunately for some of us.
Before going on, let me digress and say that fashion has nothing to do with anything that I know of.  I just had this thought and decided to put it in my blog amid the morbidly sanguine poetry that I write to someone who will not ever read it and the discussions of fundamental universe theory when I'm not turning my blog over to the cunning Wang who led the war against the US that led to china's dominance embodied in World War C and China's Weaponized economy.
Anyway, the answer is that these old designs had function.  The trench coat was designed with an eye towards survival in the harshest conditions imaginable.  The same can be said of cargo pants and sleeveless vests even thought the circumstances are different (deserts and abroad on the sea).
There were other fashions that will come and go with the whims and passions of designers, but functional clothing, particularly those tied to some romantic endeavor will always hold our imaginations.
Who doesn't want to stand on the tarmac with their one true love and let them go for some higher purpose (casablanca).  Who doesn't want to cross their dessert with a beautiful woman who wants to find herself more than the treasure on the other side?  Who doesn't see romance in the soldier, sitting in a trench trying to shield the paper from the rain and tears while writing a letter to the one who is left behind and who may never even get those words of timeless love?