There is a very good reason to use motorized over mechanized, mainly that mechanized costs 3,2 times more to produce at the lowest tier, and 4,8 times more at the highest tier. Motorized also requires no technological investment and is available day 1, so you can produce huge numbers of motorized before you even have the chance to begin producing mechanized.
Given that production costs will be the same next patch (which they will not be as far as I know), one 13 light tank III division with 7 mechanized III would cost 36,3% more than the same division template with motorized instead of mechanized, meaning you could produce around 10 motorized light tank divisions for every 7 mechanized light tank divisions.
In addition, and this is the most important part, the highest tier mechanized is already as fast as motorized in the current patch of the game.