Compare rate of hydrohalogenation between 1,2-diphenylethene and 1-phenylpropene
I would suggest that there is a potential kinetic effect here.  While the most stable carbocation formed in either case is similar, the number of collisions that result in the formation of the more stable carbocation is higher in I than in II.

If we take a very simplified view of the possible collisions in II, we can see that in a reaction with $\ce{HCl}$ only "one" of the orientations of the collisions will result in the formation of the desired carbocation. 


In the case of I, "both" of the possible collisions will lead to the formation of the same stable carbocation.


If the number of possible effective collisions is higher, the reaction will be faster.