I'm sure they considered it.
Only problem with that is if the umbilical gets cut or the trailer gets taken out by a RPG, mine, 50cal round the suit would be instant dead weight on the solder. And you can bet that the trailer becomes a primary target as soon as it come into view.
With these kinds of weapons systems it is always better to have everything in one package, saves Armour weight and reduces total target profile.
Another option might be some kind of broadcast power system like Tesla tried to do. One big mobile transmitter for a unit of XOS equipped soldiers. Of course then the transmitter becomes a primary target in any engagement but if the range was good enough it could be well back from the conflict.