Part of the problem for this is they just don't have the players anymore to carry a 3-4 defense.
In a 3-4, you need your down lineman to clog the running lanes and often times take on double teams to free up a blitzing linebacker or person from the secondary. 3-4 defenses rely heavily on a NT taking out a C/G and having two great pass rushing LBers with two solid cover LBers. Throw in someone from the secondary that can rush/be flexible and you have the ingredients for a phenomenal defense that causes matchup problems on every down. When you don't have those ingredients, just like food, the recipe can turn to ****.
90's Steelers under the 3-4.
Steed at NT
Green and Lloyd at outside LBer rushing
Kirkland at inside LBer
Woodson in the secondary
2000's Steelers under the 3-4
Hampton at NT
Harrison and Woodley at outside LBer
Timmons at inside LBer
Ike and Troy in the secondary
2000's Ravens under the 3-4
Ngata at NT
Lewis and Suggs at LBer
adequate LB partners
Reed in the secondary
The current version of the Steelers just don't have the talent (personnel, age, etc) to support a top 3-4 defense at this time. Our LB corps are the worst they've been in a long time. Woodley is a shadow of himself after the last SB run. He's definitely fatter/slower than he used to be and lost his explosiveness to beat RT's. (Plus he doesn't have awesome Aaron Smith to take out RT's anymore). Timmons is better suited as an OLB imo and was originally drafted for that according to Tomlin's quotes but was converted to ILB because of Harrison being a monster. Our secondary is old and fragile. On defense Cam Heyward has shown he has the talent to possibly be a potential playmaker but Woods not so much.
It would be nice to see primetime DS in the backfield but I don't think there are any such players coming up in the draft anytime soon