The main problem is that the Steelers have 11 players who are taking up roughly 85-87% of their total cap (prior to any restructures). That leaves $13-15% to be spread between the other 40 players under their "rule of 51". By my calculations, the Steelers top 11 players (referenced below) were making $106 million prior to these recent restructures.

Roethlisberger (age 31 in 2013) $19.6 million in 2013- $8.4 in dead money remaining
Harrison (age 35 in 2013) $10 million in 2013 - $5 million in dead money
Ryan Clark (34) $4.8 million- $1.3 in dead money
Antonio Brown (27)- $3.1 million-approx. $13.2 in future dead money- This was just restructured- I will get to this momentarily
Lamarr Woodley (turns 29 in Nov)- $13.2 million in 2013- approx. $14.2 in future dead money
Lawrence Timmons (27)- $5.9 million in 2013- $18.2 in future dead money- just restructured- will elaborate below
Troy Polamalu (32)- $10.1 million in 2013- $5.3 in future dead money
Willie Colon (30)- $7.6 million in 2013- $6.4 in future dead money
Heath Miller (turns 31 in Oct)- $8 million in 2013- $4.6 in future dead money
Ike Taylor (33)- $9.5 million in 2013- $6.4 million in dead money
Bret Keisel (35)- $4.5 million in 2013- $1.8 in dead money

So, even after the restructures to Timmons and Brown, the Steelers have $96.3 million tied up in these 11 players. That's 80% of the projected cap. That leaves them roughly $24 million to spread over the other 40 players.

The restructures to Timmons/Brown saved roughly $9 million between those two players towards 2013, which is significant, and they are still in awful shape.

If you take it a step further, Pouncey and DeCastro are making $4.3 combined next year, so now they have $100.6 tied up in 13 players.

Redman and Dwyer are both RFAs. You have to imagine that both players, given their late round/undrafted status, will have to be offered a 2nd round RFA tender, because a Round-of-draft tender would make them goners to another team. Who wouldn't give up their 6th rounder to get Jonathan Dwyer? Both players will get RFA tenders around $2 million. Now their cap is at $104.6 through just 15 players. That leaves 36 players that they now have to pay with roughly $16 million.

Suisham is on the hook for $1.7 million and Emmanuel Sanders is a RFA as well. Let's assume with him being a 3rd rounder that they offer a ROD RFA tender for $1.3 million. Sanders is a solid player worth keeping, but after 2013, where will the salary cap room to do so be found? There's another $3 million between those two. Now they have $107.6 tied up in 17 players. That leaves $13 million to be spread between 34 players ($382k per player) Obviously, that's not possible, as that puts you well below the league minimum.

Recent draft picks Cam Heyward, Jason Worilds, Mike Adams, and Ziggy Hood combine to make $5.6 million between them. All four of these players are good values at their respective salaries. Yet, the Steelers might now struggle to find room to offer them anywhere near market value as they approach free agency, much like they are unable to with Mike Wallace. If you add the $5.6 million to the picture, the Steelers now have $113.2 tied up in those 21 players. That leaves roughly $7.5 million to be spread over the other 30 players who count under the "rule of 51". That's $250k per player, or roughly half of what the lowest player on your rule of 51 roster counts against your cap. We haven't even factored in other players currently on the roster who are making above $500k/year or any incoming rookies.