It is a very sad event but what level of screening and mandatory checks would a 61 year old have gone through such that a heart attack was a surprise?.
There are a number of reasons, but short of chronic diagnosed heart disease, regardless how many preventative/clinical procedures you go through none can predict with 100% certainty you won't experience a surprise heart stoppage. Ask your doctor the next time you get a check-up what percentage his on you not having a heart stoppage in the next 6 mos. Have seen people who were a pillar of health drop dead due to a blood clot movement or other surprising issue to include 2 pilots that landed on a platform and died before they could shutdown the aircraft.