Well, most of it is rule changes. In the early days of football it was really, really hard to score. Over time this has changed quite a bit. Most of the rule changes in the past 30 years have made it much easier to score. A few examples:
- Can't hit a QB high
- Can't hit a QB low
- Can't touch a QB's helmet
- DB's basically can't touch receivers
- Back shoulder throw PI (this results in some of the dumbest calls I've ever seen...a QB can basically force PI by underthrowing a pass)
- Throw away passes past the LOS to avoid intentional grounding when outside the pocket (also a stupid rule...)
- Refs helping to facilitate no huddle hurry up offense when time is not an issue
- Holding calls on the D line to prevent O linemen from blocking LB's
- Defenses "forcing o linemen to jump" and getting called offside (this one cracks me up when they fake it)
Any more that anyone wants to add?
The defense is already at a severe disadvantage in that they don't know the play. Yet all rule changes basically make it ever harder for them to do their job. It's no wonder defenses aren't good...