The phrase 'key reason that' is correct and usable in written English.
You can use it when introducing an explanation or justification for why something happens or is done. For example: "One key reason that many people feel uncomfortable in social situations is because they lack confidence."
One key reason that makes bioactive glasses a relevant scaffold material is the possibility of controlling a range of chemical properties and, thus, the rate of bioresorption.
The results indicate that having a low initial start-up temperature is the key reason that makes the other parameters such as the current density, CO mole fraction and temperature increase rate, sensitive, especially at 108 °C.
In the pilot stage of nozzle-flapper servo valve, the flow force on the flapper is the key reason that leads to forced vibration of the armature assembly, which may result in the fatigue of the flexure tube in torque motor.
That is a key reason that Mexico is making something of a comeback these days.
Lawrence wrote of, and it is a key reason that the failure rate in counterinsurgency is so high.
This may be the key reason that the lake has also been one of Florida's best-kept secrets.
The historic neglect of viral hepatitis was also the key reason that the 2010 resolution was adopted.
This is one key reason that the process we call globalization has tended to result in widening inequality between nations.
Avoiding UV exposure is a key reason that spray tans have become so popular, but those aren't necessarily safe, either.
This growing trend was the key reason that De Leon and his RCA students became involved with Trinity.
"We've got 90percentt of the fund in the Middle East, and that was the key reason that we did well in the fourth quarter," he said.
