Because it's default configuration and (most) naive setups offer no redundancy? It is almost entirely in-memory with no easy eviction controls? If you are going to snapshot on every change because the thing you are doing needs to happen another purpose built queue like rabbitmq is going to perform better in this situation, especially during failures.
Redis can be tuned to be safer (such as persisting on every key change) but this all comes with tradeoffs.
Redis such a great piece of tech I use it first as often as I can.