That scheme might be easier to implement, I agree. It would reduce smite power by a little bit though... since one thing that it is nice to do with paladin is to wait until a critical hit to activate smite (the smite is doubled too in that case). If you are just doing normal attacks this isn't a big deal, since crits are rare in that case, but if you have something that expands the crit range (multiclass to champion for instance) and/or advantage you can get crit chance high enough that it is a good strategy.
I suppose they could add an ability called "smite on crit" and let you use that instead.
