I think the biggest issue is that there are more really good d-tackles now. Suh and Donald demonstrated how a good tackle can almost singlehandedly shut down an offense and I'm not saying that there are dudes on par with those legends, but the position has morphed from a "take up space and stop the run" to "you are the closest defender to the QB so get at him." Guys are now stronger and faster in those positions than they have ever been.
Next biggest issue is the defensive packages. Dom Capers invented the zone blitz with the Steelers back in the '90's, but it has now evolved into a package where you are seeing unexpected pressure from a fourth rusher almost every play. You no longer have to just watch for the traditional tactics, but now you have to watch to see if your guy is falling back into a passing lane or if he is assigned to a specific run gap and then if he is, holy shit you better get that head on a swivel because someone else is probably coming from behind him, oh shit, never mind, play's already over. The LB won to the outside.
Next biggest is the drop in football participation. Parents don't want their kids playing anymore and the pool of good dudes has just shrunk.
I don't think any fan base other than perhaps Michigan is particularly happy with their o-line right now.