Why should someone learn a 5 note scale before a 7 note scale? Because it has fewer notes, and the wrong notes are easier for a beginner to hear. Scales come from things called steps. A step is the distance from one tone to another. Therefore the 88 keys on a full size piano are all a HALF step apart, because no extra notes exist between each other. Scales also have WHOLE steps which always have one piano key between them.
The most commonly taught scale is the Major scale, which has 2 whole steps and a half step, plus 3 whole steps and a half step. The primary takeaway from this explanation is that since the scale has both half steps and whole steps, playing an incorrect note is more ambiguous to the ear than playing a scale with only whole steps. The greater spacing between notes makes the listener much more aware of the notes that do not belong to the scale.