Because only 1 or 2 teams hits that D every year. 30 teams in NHL, so 20 years is about right. (Been 20 years since Poti). if you think a late round steal is a top pairing, well every NHL fails from your POV. Getting even a 3rd pairing D is a win. Davidson was playing above a 3rd pairing D, so he is an even bigger win.
Look around the NHL and look at their steals from past 10-15 years (non 1st round picks)
Calgary- Brodie and Gio. None really in 20 years before that. So got their two now
Vancouver- Edler?
Winnipeg- Enstrom (Buff wasnt their pick)
Toronto- Kaberle, Stralman?
Montreal- Markov (98), Subban, Beachmin
Chicago- Keith, Buff
Minny- Spurgeon? (unsigned depth pick), Leddy?
Rangers- Girardi
NYI- Hamonic
Carolina- Faulk
Nashville- Weber, Josi, Ellis?
Tampa- Nothing (for D)
Ducks- Vat
LA- Muzzin, Voynov
Phoenix- Nothing for D (yet)
Sharks- Vlasic, Boyle
The list is far for exhaustive and really only a quick demo, could go way more in depth and missing players. But as you can see, all the "steals" are spread out over the years and over the teams. Which leads me to believe they are pure and utter luck. No team besides Nashville, Chicago and maybe Montreal has over 3 in last 15 years (Im probably missing a team here). Oilers werent going to get one with Musil, Hesketh, Bigos picks. But theyve been drafting smart with Jones/Bear/Laleggia/Paigin/ Davidson picks finally. If we get a steal, itll be pure luck. But atleast our name is in the "hat" to get a later round steal. We were always out of contention in past years with bad picks from the start