***I do not know if this has been said before, but I realized this yesterday while I was playing***
The heroes once all came under the tutelage of the heroes guild, honing their gifts, so when it was destroyed, the heroes downfall began.
This means that each generation would be weaker than the last, so the 'current' generation requires gauntlets to channel their will abilities.
Another reason could be, since the heroes may get their power from the Old kingdom (as evident by only they can use Old kingdom artifacts) the ever-increasing length of time between the destruction of the Old kingdom, and the present, the time span is causing heroes to start to not get as much... energy if you will, from the remnants of the Old kingdom.