When you build your own trailer, you can do just about anything you want as long as it is structurally sound and safely built and meets all specific state DOT requirements (usually pertaining to lights, safety chains,etc.). Therefore, in that respect, there is no reason for you to not put the door where ever you think it suits you best.
However, in the real world of commercial trailer building, due mostly to all the safety reasons previously posted (and several more) it is the law (unless that's changed in the last 20 years, which I doubt) that the main entry door on all travel trailers be on the curb side. And technically (again, unless something has changed in the last 20 years), all street side doors must be egress only, without the ability to open from the outside.
Since we (as teardroppers) are restoring, rebuilding, or replicating "vintage" trailers that were produced before these laws were put in place, we're kind of getting away with it. However (and God forbid), if anything catastrophic should ever happen as a result of someone improperly using a street side door, and a truly hardass lawyer gets their hands on the case, I'm afraid some very unwanted attention will be cast on our little trailers.
These are lessons I first learned almost 30 years ago when I thought I wanted to be the Winnebago of teardrop trailers. Reality bites, sometimes ...

...!
Just be careful, use plenty of common sense, and always be acutely aware of
everything that is happening
anywhere around you and your trailer. Safe travels, happy trails, and as always ...
CHEERS!