i think it was all a matter of cost and ability to actually cut an angled pocket. they were beginner guitars. the angled pocket in a carved top les paul is angled because the face of the guitar is at an angle. they dont use a template that is angled. you can cut the heel a lot easier if it is a straight pocket too, you dont have to worry about overshooting the angle. also to me angled necks dont look so great on flat tops that have a substantial amount of wood around the pocket. it looks perfectly fine on a v, but in my opinion it doesnt look that appealing on a lp jr singlecut. they could have very easily cut an angled pocket on the double cuts, but it was all a matter of time, cost, and effort.