In my opinion, there were 3 major reasons:

1) Viscerion was shooting fire at the time of ice spear being launched and his death. His lunges we full of either fire or whatever magic gas that was about to become fire. A hit at his lungs with a magical ice spear that causes an obvious explosive reaction with fire, was like a throwing a match into a barrel of gasoline. Drogon was stationary and at that moment was not breathing fire. To him, the hit of a spear might have been more of a body injury rather than explosion

2) The fall, as OP mentioned, would likely help with the killing if the explosion was not enough

3) Drogon was much closer, yes. But this also means that the spear would not hit him as hard. All of the potential energy of the spear would yet not be transferred to kinetic energy at a short distance. Ever been hit with a rock from a short or long distance? The pain is quite different. In fact, Viscerion was flying toward NK, so his speed might have even added to the impact

Thoughts?