since Morde can ult dragon...

Morde can't ult dragon, if he damages it and his team kills it he gets the ghost. This is to reward teamwork in controlling objectives and give the team the ability to push towers without an adc, since morde is geared toward bot with his current kit. Yorick on the other hand has an instant ult, does not replace the adc, already pushes ok and is the number 1 broken early laner who's only weakness is his late game limited helpfulness. If he were able to ult drag he would be nerfed so far into the ground he would never be picked again.

Yorick can only ult teammates, and when a teammate dies they control the revenant for a time. Yorick is basically going "hey, you might die but here's a second chance to do anything you wanted to before dying. do whatever in the time you have". Mordekaiser ults enemies, and when they die Mordekaiser controls the enslaved soul. Mordekaiser is going "I'm controlling you, now shut up and start murdering who I say".

Mordekaiser taking control of dragon's soul fits. Yorick doing so does not; the dragon, still being all free to do whatever, would just have an extra 8 or 12 or whoever many seconds to continue attacking yorick and the guys who killed him.