simply because it is useless!
any coin or token that has no use cases is eventually going to end up in the same place as the other couple of thousand altcoins that have come along and died in a short time.
if that is not enough then your own statement should do it:
ICON still gives 12% monthly by staking it,
any coin that gives this much away creates a lot of circulating supply that people gained easily and they will obviously dump. it is a pretty simply supply and demand thing!
no usage=no demand + high circulating supply + free high profit percentage => declining price