Regioselectivity of electrophilic addition of Halogen and Water (Halohydrin) on Alkenes?
This is not a 100% answer, but it was too detailed to fit in the comment section.
Following up from the comment, here's a reference from Inorganic Chemistry by Morrison and Boyd:

There is evidence, of a kind we are not prepared to go into here, that these compounds are not formed by addition of preformed hypohalous acid, $\ce{HOX}$, but by reaction of the alkene with, successively, halogen and water.

Halogen adds (Step 1) to form the halonium ion; this then reacts, in part, not with the bromide ion, but with water (Step 2) to yield the protonated alcohol.
Propylene gives the chlorohydrin in which chlorine is attached to the terminal carbon. This is typical behaviour for an unsymmetrical alkene; orientation follows markovnikov's rule, with positive halogen going to the same carbon that the hydrogen of a protic reagent would. Now, this orientation would be perfectly understandable if the intermediate were an open carbocation: the initial addition of halogen yields the more stable carbocation - secondary, in the case of propylene. But the stereochemistry indicates that the intermediate is not an open carbocation, but a cyclic halonium ion.

And that's all. Quite anticlimatic, as I was expecting a reason for the regioselectivity. Most other references and papers I have seen online also do not have a concrete explanation for the regioselectivity, and they resort to hand-wavy explanations involving resonance for the same. This reaction mechanism is also quite similar to Oxymercuration-demercuration, whose regioselectivity is again explained by the &quot;three resonating structures&quot;. I could not find diagrams of these resonating structures, and while this question asks the same, the resonance structures or the intermediate are not properly explained in the answer.
My best suggestion for you would be to accept the explanation in your textbook as that is adequate for now. Until someone more knowledgable posts a better answer with references, that should suffice.