What is most likely the reason for Edward the fifthand Richard's disapearance?

No one really has any idea. The general assumption was that they were murdered, but by who is anybody's guess.

The leading suspect is Richard III, who is thought to have killed the princes to secure his hold on the throne. However, at the time the princes disappeared, Edward V's claim to the throne had been rejected by Parliament, due to his parents not being legally married. The same act of Parliament ("Titulus Regius") elevated Richard from Lord Protector for Edward V to king. At that point, Richard would have no need to kill the princes to secure his claim to the throne. Of course, he could have done so anyway, just to be safe. But there's further evidence that at the time of Edward V's ascension, Richard III made significant efforts to secure the safety of the princes.

The other likely candidate is Henry VII. Assuming that the princes survived the fall of Richard III, Henry would have good reason to dispatch them. Henry had almost no claim to the throne at all. Once we took power, he set about executing all possible challengers to his throne. Further, Henry had "Titulus Regius" repealed and ordered that all copies of it be destroyed. The only practical reason to do this was to remove Richard III as the legitimate king (posthumously) and replace him with Edward V. Thus, Henry could build a more likely case for pinning the princes' murders on Richard (which really didn't make much sense if he was already the legitimate king).

There is no real consensus on the issue among historians, but IMHO Henry's actions in re Titulus Regius combined with his prosecution of other candidates make him the much likelier suspect.