Why secondary alkyl groups are more prone to shifting than primary?

Why secondary alkyl groups are more prone to shifting than primary? I attach example from Grossman book [p. 20]:



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> [![enter image description here](https://i.stack.imgur.com/64FhI.png)](https://i.stack.imgur.com/64FhI.png)
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> When $\ce{R} = \ce{CH2CH2Ph}$, the coordinated $\ce{Al}$ simply transfers a $\ce{Me}$ group to the carbocation $\ce{C}$ (σ bond nucleophile). The $\ce{O}$ atom then coordinates another equivalent of $\ce{AlMe3}$ before the product is obtained upon workup.
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> [![enter image description here](https://i.stack.imgur.com/8AX3b.png)](https://i.stack.imgur.com/8AX3b.png)
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> When $\ce{R} = \text{cyclohexyl}$, the $\ce{R}$ group migrates (1,2-alkyl shift) to give a new carbocation. (2° Alkyl groups are more prone to migrate than 1° alkyl groups.) After $\ce{Me}$ transfer to the new carbocation and coordination of another equivalent of $\ce{AlMe3}$, workup gives the product.
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> [![enter image description here](https://i.stack.imgur.com/rdqWf.png)](https://i.stack.imgur.com/rdqWf.png)
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This is quoted from "The Art of Writing Reasonable Organic Reaction Mechanisms", Second Edition by Robert B. Grossman, DOI: 10.1007/b97257.


