For me there is a simple reason; it is not controlled by the governments. As someone who comes from a nation with corrupt officials, fiat means that there will be people at the top who will give themselves a lot, and make the money less worthy, so the money I own will worth less and less, while they don't have to worry about that because they get rich faster than the inflation rate because they can print more for themselves.

This is why, I moved to bitcoin because they can't ruin this. Doesn't mean big nations can't, USA or China may try, but my nation is so small and corrupt that they can't even ruin Zimbabwe, let alone ruin bitcoin so I have no worries about it.