I think this is very implied to be canon, but I am throwing this into the world anyway.
Elemental masters can only give up elements when they're ready to or physically unable to continue or seen as unfit to continue their duties by their sources, and they can choose who the element goes to if they have no children or family remaining.
Shade only gave his up once he saw that Obscuria was old enough to handle the responsibility.
Ray and Maya gave up their powers a few years after their capture in the hopes that Nya and Kai would have a fighting chance against Krux if he ever came back for them.
Hiberus gave up his powers to Zane before his death.
And Morro (+Plus many others) being unable to continue their duties because they are dead.
The elemental master can as well just choose to not give up their element to anyone and die with them (Time twins), but their sources will eventually choose a new holder of the element, although it takes a lot more time to do this.
ough i lov this idea because it means elemental masters most often have to make the deliberate and difficult choice to part with something that has represented their fighting spirit for likely a majority of their lives. and having to make the choice to hand it off to someone is an even more intimate choice with having to decide who is fit to handle something great and turn it into something greater
the ray and maya one breaks my heart too, as well as morro's because i imagine him being the master of wind was one of the only aspects about himself that he could be proud of until he realized what he didn't accomplish much with it besides hurting people
