Many reasons;
1. Oro was a noob with edo tensei
2. He hasnt done his research enough on them to use their abilities to the fullest
3. Oro is an idiot who underestimates his opponents. If i remmeber correctly he stated that he wanted to enjoy hiruzen being raped by his 2 former teachers.
4. Is the most important one which is plot. If prime hiruzen was to beat either the 1st or second hokage he would take it with stupidly high difficulty. There is no way in hell, old hiruzen would beat either hokage's if they were at their strongest. Kishi had to find a way to weaken them enough for old hiruzen's 'victory' to look reasonable. If the 2 hokages were truly at their strongest, old hiruzen wouldve been beaten EASILY and that will lead to the destruction of konoha, because nobody is stopping those 2 hokages. I mean in the village alone it was said that there was nobody capable of stopping orochimaru, so imagine those 2 hokages + oro. The series obviously has to go on and so does konoha therefore kishi created a scenario where the hokage's power levels were so low that hiruzens 'victory' over them was reasonable.
Do however believe that prime hiruzen would beat either at their strongest with ridiculously high difficulty