Macbeth thinks he should be king of Scotland because he believes the witches’ prophecies. He frames Malcolm and Donalbain for their father’s murder so he can take the throne.
Malcolm and Donalbain are suspicious immediately. They believe that whoever killed their father will be coming for them next. So they flee.
This murderous shaft that's shot
Hath not yet lighted, and our safest way
Is to avoid the aim. (Act 2, Scene 3, p. 37)
When Macduff tells Ross that Malcolm and Donalbain have fled to England and Ireland, Ross is immediately annoyed.
Thriftless ambition, that wilt ravin up
Thine own life's means! Then ’tis most like
The sovereignty will fall upon Macbeth. (Act 2, Scene 4, p. 39)
Since the two are not in the kingdom, and since they are suspected in their father’s death, they cannot claim the throne. As a cousin, Macbeth is the next in line. As we know, Macbeth was surprised when he was not named Duncan’s successor.
See here for quotes: http://www.enotes.com/macbeth-text