I actually asked this question of Krause and they sent me to their expert a gentlemen by the name of Don Bailey. he has a web site
http://www.donbailey-mexico.com. Essentially his answer was it has been that way for a long time and it was his assummption that many were destroyed and made them more rare. But his exact qyuote was:
"The 1987 $5 have always been scarcer then any of the other years. Causes for this could be several factors, and these are just guess as to what cis the cause. One could be that when they were struck there was enough in the system and they didn't need them to supply the banking system. They may have set in the mint and not get into the system. What happened to them is unknown, melted? setting on the floor in one of the many mint or Banco vaults
In the late 80s the Mexican roll collecting craze was over so few were saved by collectors.."