Because the path to ground has been interupted. Usualy the cause is a faulty 120VAC receptacle in your garage connection or the flimsy extension cord you are using.
The only way your trailer frame gets a ground is though the shore power plug.
Use one these 120VAc circuit testers from LOWES on the 120VAC receptacle before plugging in your RV30A-15A long dogbone adapter and it will tell you whether your connection is good or bad...
Don't take this lightly - what may be just a tingle to you might really do harm to someone else...
Roy Ken