Since Robinson's day, shoulder pads have gotten smaller, knee and thigh pads have been ditched. Some guys today don't even wear a cup.
Defensive players have been ditching or downsizing any equipment they think detracts from burst, speed and agility that contemporary "finesse" defenses demand for combating the passing game (and now the run-option threats). And they have to keep up with the offensive players who have likewise lightened their load.
Downsized protective gear is a contributing factor to poor tackling technique.
The league is making knee and thigh pads mandatory starting this season in an attempt to curb some injuries. Players who fail to wear them will not be allowed to take the field. As a byproduct, it's may be one small step in the right direction for improved tackling.
In the end, though, the shrunken shoulder pads are more problematic from a fundamental tackling standpoint. The ones in Robinson's day were huge by today's standard; guys were tackling pad-on-pad...now it's closer to bone-on-bone.