One subtle but important reason to use cross-region failover is that the network latency between the regions can prevent many casual or accidental dependencies

One subtle but important reason to use cross-region failover is that the network latency between the regions can prevent many casual or accidental dependencies between instances (if you configure instances in two regions to use the same database server, latency can cause the distant region to perform poorly). This is why it's really hard to get cross region failover to work. Because you really need to make them independent.