For years you could blame it on drafting CBs late. They always seemed to be drafting CBs in rounds 4-7 and you can't really fault a team for not finding quality starting CBs late in the draft.
The last few year though, they have stepped it up and it hasn't really been better. Here's a list of all DBs drafted under Tomlin with those drafted in the first 3 rounds in Bold
2007 - William Gay CB 5
2008 - Ryan Mundy S 7
2009 - Keenan Lewis CB 3, Joe Burnett CB 5
2010 - Crezdon Butler CB 5,
2011 - Curtis Brown CB 3, Cortez Allen CB 4
2012 - Terrence Frederick CB 7
2013 - Shamarko Thomas S 4, Terry Hawthorne CB 5
2014 - Shaq Richardson CB 5
2015 - Senquez Golson CB 2, Doran Grant CB 4, Gerod Holliman S 7
2016 - Artie Burns CB 1, Sean Davis S 2,
2017 - Cameron Sutton CB 3, Brian Allen CB 5
2018 - Terrell Edmunds S 1, Marcus Allen S 5
Clearly they made DBs a higher priority starting in 2015 and so far, not much to show
Golson was a bust
Burns has had moments but has mostly been a bust
Sean Davis has been OK and still has potential but hasn't broken out yet
Cameron Sutton hasn't shown much
Edmunds and Allen too early to tell
I don't know if it's a scouting issue or a coaching issue, but the fact is the steelers have not drafted a better than average DB since Bryant McFadden in 2005 and they haven't drafted an actual pro bowl caliber player since Ike and Troy in 2003
At this point, they should get a good DB by shear luck of the draw after so many misses.