Talked about this in-depth too in the book.
Short answer - the circus preventing home games in SCF's, the way the draft rights were at the time, hockey not being MSG's priority, and World War II, where really, it took the Rangers nearly 20 years to recover after WWII, especially after Lester Patrick sold off the team's prospects/minor league players in his wrong anticipation of all hockey being stopped during wartime.
In addition, look when the O6 era started - World War II, which ended the Brooklyn/NY Americans franchise. As the "southernmost" team in the league, the Rangers took the short end of it too.
Post O6 era, you had the two crucial injuries in the 70's - Ratelle and Nilsson.
From 2000 on, you had Glen Sather running the team, who to this day, is not only the longest-tenured GM in all of NYC sports to never win a championship, but remains the longest-tenured front office employee to never win a championship in NY too. Dolan loves the guy, and since NYR is the most profitable team in the league, never cut bait on Sather, despite all of Sather's grand proclamations and failure to win a Stanley Cup in NY.
