Why older bricks have greater clutch power?
This is a partial answer (or theory).
The moulds for most of the basic bricks have been changed over the years to use less plastic while (hopefully) retaining clutch.  

2 x N bricks were given thinner walls with 'notches' for the studs. This may have slightly weakened clutch power.

  

1 x N bricks (and some plates) were given holes through the bottom tubes and had some supporting walls removed. Not sure how this might've affected clutch power though.



Photos taken from here.