Question

Why are minor resonance forms less commonly occuring?

Answer 1

The least stable resonance "forms" occur least often. Stability corresponds to the capacity of a compound to maintain its current state. The more stable the resonance structure, the lower the energy, and the easier it is to maintain that structure.

Resonance structures are essentially "stills" of the molecule, whose structure oscillates between "structural constructs" due to electron delocalization. These structural constructs are as if we let all of the delocalized electrons move absolutely into the next orbital, tracked when the first double bonds at each location were formed as a result of that, froze the electrons and orbitals, then captured the "still".

If any of these "stills" (resonance structures) is less stable than another, the less stable one occurs less often because it is able to be maintained for less time.

Organic compound stability tends to be determined by the location of a charge relative to the nearest `pi` electrons (for resonance stabilization), protons (for 1,2-hydride shifts), or alkyl groups (for 1,2-alkyl shifts). For example, a vinyl cation like this is less stable:

...than a similar allyl cation:

You can tell that an allyl cation has the capacity for resonance stabilization, but a vinyl cation might not.