Why does hexokinase bond phosphate to the hydroxyl group on carbon-6 of glucose in glycolysis?
First of all some nitpicking, but the actual phosphorylation happens not to the Carbon-6 but to the hydroxyl group bound to this carbon atom. The mechanism is a nucleophilic attack of the hydroxyl group on the terminal phosphoryl group of ATP. Schematically, this looks like this (image from here):



The main question, why the reaction happens on this carbon atom can be answered rather simple (although the explanation is a bit longer): Because only this is openly available for a chemical reaction and the others are inside the hexokinase and thus shielded.

Hexokinase exists in two forms: One relaxed, open when no substrate is bound and one closed with a changed morphology when the enzyme has bound its substrate. This mechanism is called "induced fit" and a quite common motif in enzyme catalyzed reactions and brings together the active center of the enzyme, possible co-factors and the substrate and excludes interfering other actions. 

In the example of the hexokinase, glucose is brought into close proximity to the terminal phosphoryl group of enzyme-bound ATP to enable the chemical reaction. Because all other potential hydroxy groups are shielded by the enzyme, only the group at the carbon-6 can be chemically active. For more details, see the references.

Schematically, this looks like this, with the open form to the left (the arrow indicates the binding site for glucose) and the closed form right (image from here):



References


The Structure and Mechanism of Hexokinase
Mechanism of liver glucokinase.