Here is the story:
Merck and Schering-Plough were both very good companies. The issue was scalability. The upper management screwed it up. They never saw the big picture. They did not understand how to manage all the moving parts. They had no clue what they were doing and never provided middle management any guidance. People were willing to work hard. The issue was people did not know what to do. Everything was kind of in a holding pattern. Projects were being shuffled all over the place. There was no commitment from upper management to move forward. People who had ideas were seen as a threat. I think it was more because they just got tired of doing busy work.
Culture:
Schering-Plough employees were the nicer people. They were not prepared and could not handle how Merck's culture. They got bum rushed. They really had no chance. Merck management really knows how to play the game, change the rules during the game, and somehow still play the victim.
If Merck and SP played a football game, somehow all the refs would also be from Merck. A Merck receiver would catch the ball at the 50 yard line 5 yards out of bounds and the ref would call it a reception. SP would argue the call and get a flag for unsportsmanlike conduct. Then the Merck refs would call it a touchdown and eject the SP players. It was a no win situation.
All that would have never happened if upper management understood anything about scale and guidance. They were the ones that caused the culture clash. Middle management and lower were lost once the ink dried on the merger. Both Merck and SP upper management failed.