The truth is there isn’t an awful lot of difference between the artist frameworks. Actix-web was one of the first though, so the reason to choose it over others would be that it’s a bit more built out feature wise.
Actix-Web actually differentiates itself from most other Rust servers in that it runs multiple per-thread runtimes as opposed to a single work-stealing runtime. This probably doesn't matter to most, but if you really care about latency, it might be a factor to consider.