There is still no proof for this but some scientists suggest people over 80 years old came in contact with a similar virus when they were young and now their immune system is overreacting (like when the second sting of a wasp can be much worst that the first).

In the Netherlands they found out by coincidence that some people who have a broken TLR7 gene don't fight the virus so the virus hits them very hard.

So there may be multiple reasons.